PRINCIPLES  OF  THE  TELEPHONE 


PART  I 
SUBSCRIBER'S  APPARATUS 


McGraw-Hill  DookCompatry 


Electrical  World         The  Engineering  andMining  Journal 
En5ineering  Record  Engineering  News 

Railway  Age  G azettx?  American  Machinist 

Signal  E,ngin<?<?r  American  Engineer 

Electric  liailway  Journal  Coal  Age 

Metallurgical  and  Chemical  Engineering  Power 


Transmitter 


Battery 


Receiver 


Induction  Coil 


m 


Generator 


HookSwitch 


Contact 


Crossing  of  Wires 
not  Joined 


Jack 


Impedance  Coil 


Conde™r 


"C^O"  ^ 


1  Ground 

Junction  of  Wires 


Symbols  used  in  diagrams. 


Electromagnet 


Frontispiece 


INDUSTRIAL  EDUCATION  SERIES 

PRINCIPLES 
OF  THE  TELEPHONE 

PART  I 
SUBSCRIBER'S  APPARATUS 


PREPARED  IN  THE 

EXTENSION  DIVISION  OF 
THE  UNIVERSITY  OF  WISCONSIN 


BY 


CYRIL  M.  JANSKY,  B.  S.,  B.  A. 

ASSOCIATE   PROFESSOR   OP   ELECTRICAL    ENGINEERING 
THE   UNIVERSITY   OP   WISCONSIN 

AND 

DANIEL  C.  FABER,  E.  E. 

ASSISTANT   PROFESSOR   OP   ELECTRICAL  ENGINEERING 
THE   UNIVERSITY   OP  WISCONSIN 


McGRAW-HILL  BOOK  COMPANY,  INC, 

239  WEST  39TH  STREET.     NEW  YORK 


LONDON:  HILL  PUBLISHING  CO.,  LTD. 

6  &  8  BOUVERIE  ST.,  E.G. 

1916 


COPYRIGHT,  1916,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


IHK  MAPI.E  PRESS  YORK  PA 


PREFACE 

In  order  that  this  text  might  appeal  to  and  be  of  practical  use 
to  men  who  are  actively  engaged  in  the  installation,  care,  and 
operation  of  telephone  apparatus,  every  advantage  of  contact 
with  the  men  engaged  in  the  industry  was  utilized  by  the  authors, 
including  conferences  especially  with  the  engineers  of  the  Wis- 
consin Telephone  Company,  from  whom  many  valuable  sugges- 
tions were  received. 

Although  details  of  construction  are  not  given  to  any  extent 
it  was  the  purpose  of  the  authors  to  clearly  set  forth  the  principles 
that  underlie  good  construction.  The  main  emphasis,  however, 
is  placed  upon  the  principles  of  operation  of  different  types  or 
makes  of  subscribers'  apparatus,  together  with  a  discussion  of 
methods  of  locating  faults  and  their  correction. 

This  is  the  first  part  of  the  course  and  is  mainly  confined  to 
subscribers'  apparatus.  The  two  other  parts  to  follow,  which 
are  in  preparation,  will  treat  of  central  office  equipment  and  out- 
side construction. 

The  authors  wish  to  acknowledge  their  indebtedness  to  manu- 
facturers of  telephone  apparatus  for  their  unfailing  courtesy  in 
furnishing  illustrative  material  and  data.  They  are  also  under 
great  obligation  to  Mr.  L.  Killam,  H.  L.  Miller,  and  F.  J.  Mayer 
of  the  Wisconsin  Telephone  Company  for  advice  and  suggestions, 
and  especially  to  the  latter  for  reading  the  manuscript. 

C.  M.  J. 

D.  C.  F. 


vu 

342914 


CONTENTS 

PREFACE vii 

CHAPTER  I 
INTRODUCTORY 

ART.  PAGE 

1.  Historical 1 

2.  Telephone  Operation 1 

3»  Telephone  Instruments 3 

4.  The  Transmitter 3 

5.  The  Receiver       ...'., 4 

6.  The  Generator 4 

7.  The  Ringer      4 

8.  The  Hook  Switch ' 4 

9.  The  Induction  Coil 4 

10.  The  Battery 4 

CHAPTER  II 

ELEMENTARY  ELECTRICAL  PRINCIPLES 

11.  Primary  Batteries .  6 

12.  Electrical  Pressure 7 

13.  Telephone  Batteries 7 

14.  Conductors  and  Insulators    . 8 

15.  Electrical  Resistance      9 

16.  Resistivity 9 

17.  Wire  Measurement 10 

18.  Gage  Numbers 11 

19.  Units  of  Resistance 13 

20.  Unit  of  Electrical  Pressure 13 

21.  Electric  Current 14 

22.  The  Ampere 15 

23.  Pressure,  Current,  and  Resistance  . 15 

24.  Electric  Circuits .  15 

25.  Series  Circuit 16 

26.  Parallel  Circuit 16 

27.  Closed  Circuit 16 

28.  Open  Circuit    . 16 

29.  Short  Circuit  .    .    .    . 16 

30.  Grounded  Circuit 17 

31.  Resistance  of  a  Series  Circuit '.  17 

ix 


x  CONTENTS 

ART  PAGE 

32.  Resistance  of  a  Parallel  Circuit 17 

33.  Cells  in  Series      20 

34.  Cells  in  Parallel .  20 

35.  Battery  Resistance  for  Parallel  Connections 21 

CHAPTER  III 
MAGNETIC  PRINCIPLES 

36.  Receiver  Action 23 

37.  Magnetism 23 

38.  Magnetic  Substances 24 

39.  Magnetic  Induction 24 

40.  Experiment  1 24 

41.  Magnetic  Action 25 

42.  Experiment  2  .....' 25 

43.  Laws  of  Magnetic  Attraction  and  Repulsion 25 

44.  Experiment  3 26 

45.  Permanent  and  Temporary  Magnets      26 

46.  Experiment  4 27 

47.  Magnetic  Lines 27 

48.  Experiment  5 28 

49.  The  Magnetic  Circuit 28 

50.  Electromagnetism 29 

51.  Experiment  6 29 

52.  Solenoids 29 

53.  Experiment  7 30 

54.  Electromagnets 30 

55.  Horseshoe  Electromagnet 32 

56.  The  Ironclad  Electromagnet 32 

57.  Construction  of  Electromagnets 32 

58.  Magnet  Wire       33 

59.  Magnetic  Action  of  Receiver 33 

CHAPTER  IV 

SOUND 

60.  Sound " 35 

61.  Velocity  of  Sound       36 

62.  Properties  of  Sound 36 

63.  Pitch 36 

64.  Loudness 36 

65.  Timbre  or  Quality 36 

66.  Transmission  of  Speech 37 

67.  Experiment  8 , 37 

68.  Variable  Resistance    .  38 


CONTENTS  xi 

CHAPTER  V 

TRANSMITTERS 
ART.  PAGE 

69.  The  Carbon  Transmitter 40 

70.  White  Solid-back  Transmitter "...  40 

71.  New  Western  Electric  Transmitter 43 

72.  Kellogg  Transmitter 44 

73.  Monarch  Transmitter 45 

74.  Operator's  Transmitter 45 

75.  Carbon  Electrodes 45 

CHAPTER  VI 

RECEIVERS  AND  INDUCTION  COILS 

76.  The  Receiver       ,    .V.    ........  48 

77.  Early  Receivers .    .    .   y 48 

78.  Induced  Electric  Pressure ".  . ,    .    .  ,; 48 

79.  Direct  Current :    .    .    ....  49 

80.  Alternating  Currents ;.    ...    .    .    ....    .    .    .  49 

81.  Experiment  9 .;,........  49 

82.  The  Receiver  as  a  Transmitter     .    .    .    ,    .    .    ,    . 50 

83.  Bipolar  Receiver .......;. 50 

84.  Western  Electric  Receiver    .    .    .    .    .    .  '.'.'..  V 51 

85.  The  Kellogg  Receiver    .    .    .    .'.....'...    .  V V  .    ...  53 

86.  Operator's  Receiver •    •    •    •    - * * ••    -    .'    .    •    ...  55 

87.  Sensitiveness  of  Receivers . •• ;    .'.-.-.    ...  55 

88.  Direct-current  Receiver     .    . .    .;...-.    .  56 

89.  The  Automatic  Electric  Co.'s  Direct-current  Receiver 56 

90.  The  Monarch  Direct-current  Receiver 57 

91.  Self-induction      57 

92.  Self-inductance 58 

93.  Mutual  Induction  ......   ^ 58 

94.  Impedance •    ~ 59 

95.  The  Induction  Coil    .    .    .    ,    .    .    .  ' 59 

CHAPTER  VII 

SIGNALLING  APPARATUS  AND  CIRCUITS 

96.  Signalling  Circuits      .    r 63 

97.  Generators 63 

98.  The  Telephone  Generator 66 

99.  Automatic  Switch 68 

100.  The  Ringer 69 

CHAPTER  VIII 

THE  SUBSCRIBER'S  TELEPHONE  SET 

101.  The  Complete  Telephone      72 

102.  The  Hook  Switch   .  72 


xii  CONTENTS 

CHAPTER  IX 
LOCAL  BATTERY  SYSTEMS 

ABT.  pAGE 

103.  Classification  of  Local  Battery  Systems 76 

104.  Series  Telephone  System 76 

105.  Local  Battery  Circuit 78 

106.  The  Bridging  Telephone 78 

107.  Connections  of  Bridging  Telephone 79 

108.  Telephone  Instruments 80 

109.  Standard  Wall  Set      80 

110.  Hotel  Set 82 

111.  Desk  Set 83 

CHAPTER  X 
COMMON  BATTERY  TELEPHONES 

112.  General , 87 

113.  The  Condenser 88 

114.  Manufacture  of  Telephone  Condensers  ...:........  90 

115.  Analogy  for  a  Condenser 92 

116.  Action  of  a  Condenser 93 

117.  Function  of  Condenser  in  Telephone  Circuit 95 

118.  Receiver  and   Transmitter  in  Series;  Condenser  and  Ringer  in 

Series 95 

119.  Induction  Coil,  No  Condenser  in  Receiver  Circuit 96 

120.  Induction  Coil  and  Condenser  in  Ringer  and  Receiver  Circuits    .  96 

121.  Retardation  Coil  in  Place  of  Induction  Coil 98 

122.  Wheatstone's  Bridge  Connection 99 

123.  C.  B.  Wall  Sets 100 

124.  Hotel  Sets 100 

125.  Desk  Sets                                                                                                 .  101 


CHAPTER  XI 

FAULTS  IN  SUBSTATION  TELEPHONE  APPARATUS 

126.  General 106 

127.  O.  K.  or  Correct  Tests,  Local  Battery  Telephones,  Line  Discon- 

nected    106 

128.  Side  Tone 107 

129.  Classification  of  Faults      107 

130.  Fault  Finding,  Local  Battery  Telephones,  Substation  Apparatus  110 

131.  Faults  in  Central  Energy  Substation  Instruments Ill 

132.  Circuits  of  C.  B.  Subscribers'  Telephones      112 

133.  Locating  Faults  in  C.  B.  Telephones 113 


CONTENTS  xiii 

CHAPTER  XII 
PROTECTION  OF  TELEPHONE  LINES  AND  APPARATUS 

ART.  PAGE 

134.  Need  for  Protection 117 

135.  Sources  of  Excessive  Voltage        117 

136.  Heating  Effect  of  Current 117 

137.  Lightning  Phenomena * 118 

138.  Lightning  Conductors 120 

139.  Lightning  Arresters 121 

140.  Carbon  Block  Arresters     .    .    .    .    ."•-.- 122 

141.  Self -cleaning  Arresters ...    .    .    .    .    .    .  124 

142.  Location  of  Lightning  Arresters ..,-.....  125 

143.  Protection  against  Power  Circuits      .    .    .    ...    ....    .  ..    .  125 

144.  Fuses ".  /  .    .....    v  ..    .    .    .   .    .    .  126 

145.  Protectors ,.    ...  V  .<•...'..'..  127 

146.  Protection  against  Weak  Currents 128 

147.  When  Substations  Need  Protection '   .    .  • , 130 

CHAPTER  XIII 

N, 

INSTALLATION 

148.  Entrance  Holes  .....    ...,,,.    ...    ......    .    .    ...    .  132 

149.  Leading-in  Wires    .......    . .  ;    .    .    .    .    .    .    .'.    .    .    .    .  132 

150.  Location  of  Protector     .    .....   ,    .  ".  «.  ...    .  -.  v.    .    .:;  .    .    .  133 

151.  The  Inside  Wiring      ....    ...    .    .    .    .    .    .   1    ...    .    .    .    .  133 

152.  Ground  Wiring ...    ...........  134 

153.  Location  of  Telephone  Set ".    .    .....    ,.,...    .  135 

CHAPTER  XIV 
PARTY  LINES 

154.  Definition .    .^v   .....   *    .    /.    .    ....  137 

155.  Classification  of  Party  Lines    ....                                          .    .  137 

156.  Code  Ringing      .    .    .    .....    ...                                            .  ,  138 

157.  Selective  Ringing    ....,...,„ 140 

158.  Harmonic  Ringing      .   ......  ^.   .. 142 

159.  Extension  Bells  .    .    ...    .    .                               144 

CHAPTER  XV 

INTERCOMMUNICATING  TELEPHONE  SYSTEMS 

160.  Definition .  146 

161.  Common  Battery  Interphone  Systems .147 

162.  Western  Electric  Intercommunicating  System      148 

163.  The  Kellogg  Intercommunicating  System      153 

164.  The  Monarch  Intercommunicating  System 154 

INDEX            * 157 


PRINCIPLES  OF  THE 
TELEPHONE 

CHAPTER  I 
INTRODUCTORY 

1.  Historical. — The  first  mention  of  the  transmission  of  speech1 
to  a  considerable  distance  probably  was  by  Robert  Hooke  in  1667 
who  described  how  he  had  transmitted  sounds  through  a  con- 
siderable distance  by  the  aid  of  a  tightly  stretched  string.     Later 
developments  of  Hooke's  method  of  transmitting  sound  show  the 
substitution  of  a  wire  for  the  string  in  the  original  experiments. 
In  any  case  the  sounds  of  speech  could  be  transmitted  only  a  few 
hundred  feet,  and  it  was  not  until  use  was  made  of  electricity 
that  the  telephone  became  a  commercial  possibility. 

The  electric  telephone  was  patented  by  Alexander  Graham 
Bell  in  1876,  and  the  first  public  exhibition  of  it  was  made  that 
year  at  the  Centennial  Exposition  in  Philadelphia.  Since  that 
date  the  number  of  telephones  has  grown  rapidly;  in  fact,  it  is 
doubtful  if  any  other  invention  can  show  such  a  rapid  commercial 
development.  At  present  it  is  not  only  possible  to  carry  on  a 
conversation  between  New  York  and  San  Francisco  by  wire, 
but  between  New  York  and  Honolulu  by  wireless  telephony. 

2.  Telephone  Operation. — The  modern  telephone  system  con- 
sists of  subscribers'  instruments,  the  central  office,  and  the  con- 
necting lines,  so  arranged  that  any  subscribers'  instrument  can 
be  connected  at  will  to  any  other  instrument  of  the  system.     It 
is  in  the  central  office  that  connections  between  subscribers'  lines 
are  made,  a  switchboard  in  which  the  lines  terminate  being  lo- 
cated at  this  place.     In  the  manually  operated  system,  which 
will  be  the  only  one  considered  at  present,  switchboard  or  central 
operators,    who   make   the   desired   connections   by   hand,    are 
provided. 

The  person  making  the  call  first  signals  the  central  operator 
by  turning  the  crank  of  the  telephone  generator  in  the  magneto 

1 


2  PRINCIPLES  OF  THE  TELEPHONE 

system.  Turning  this  crank  causes  an  electric  current  to  flow 
through  the  wires  to  the  central  office,  where  it  operates  a  signal, 
showing  the  operator  that  a  connection  is  desired.  As  soon  as 
the  subscriber  has  signalled  the  central  office  he  removes  the  re- 
ceiver from  the  hook  and  listens  until  the  operator  has  answered 
his  signal,  when  he  tells  the  operator  the  desired  number.  The 


FIG.  1. 

operator  then  connects  the  line  of  the  calling  subscriber  to  the 
line  having  the  number  for  which  he  called. 

The  next  step  is  to  attract  the  attention  of  the  called  sub- 
scriber, which  is  done  by  ringing  his  telephone  bell.  As  soon 
as  the  wanted  subscriber  answers  his  call  by  removing  the  re- 
ceiver from  the  hook,  the  connections  between  the  two  instru- 
ments are  complete,  and  the  two  subscribers  can  talk  with 
each  other. 

In  order  to  understand  the  manner  in  which  the  sound  of 
speech  made  at  one  end  of  a  telephone  system  is  reproduced  at 


INTRODUCTORY 


the  other  end,  it  will  be  necessary  first  to  get  rid  of  the  popular 
idea  that  the  sounds  produced  at  one  end  of  the  wire  actually 
travel  to  the  other  end,  for  such  is  not  the  case.  Speech  is 
transmitted  electrically.  The  sound  waves  of  the  voice,  at  the 
transmitting  or  sending  station,  set  up  fluctuating  electric 
currents  which  pass  over  the  line  and  cause  a  diaphragm  at  the 
opposite  end  to  vibrate  as  these  currents  fluctuate,  thus  repro- 
ducing as  nearly  as  possible,  by  means  of  the  vibrations  of  the 
receiver  diaphragm,  the  original  sounds. 


FIG.  2. 

3.  Telephone  Instruments. — In  Figs.  1  and  2  are  shown  the 
main  working  parts  of  a  common  form  of  subscribers'lnstrument. 

4.  The  Transmitter.— The  transmitter,  as  its  name  indicates, 
is  used  for  transmitting  or  sending  the  message.     The  working 
parts  of  the  transmitter  consist  of  a  thin  iron  diaphragm  about 
2%  in.  in  diameter,  and  two  carbon  disks  about  %  in.  in  diame- 
ter separated'  by  a  small  quantity  of  granulated  carbon.     One 
of  the  carbon  disks  is  attached  to  the  iron  diaphragm,  which 
is  caused  to  vibrate  by  the  sound  waves  of  the  voice  when  the 

2 


4  PRINCIPLES  OF  THE  TELEPHONE 

transmitter  is  in  use.  The  variations  in  the  pressure  between 
the  disks  cause  the  electric  current  flowing  through  the  trans- 
mitter to  vary  from  time  to  time,  thus  sending  fluctuating  cur- 
rents over  the  line.  The  details  of  construction  and  operation 
of  the  transmitter  are  taken  up  in  a  later  chapter. 

5.  The  Receiver. — The  receiver  consists  of  a  shell,  usually 
made  of  hard  rubber,  containing  an  electromagnet  and  a  thin 
iron   diaphragm.     The   fluctuating   electric    currents   from   the 
line  flowing  through  the  coil  of  the  electromagnet  cause  the  force 
with  which  the  magnet  attracts  the  diaphragm  to  vary  from 
time  to  time,  thus  causing  the  diaphragm  to  vibrate  and  give 
out  sounds. 

6.  The  Generator. — The  generator  is  a  small  dynamo  and 
operates  only  while  the  crank  is  being  turned.     It  is  not  used 
during  conversation  over  the  telephone,  but  is  used  to  signal  the 
central  office  when  a  connection  is  desired  by  the  subscriber. 

7.  The  Ringer. — The  ringer  is  an  electric  bell  which  is  used 
to  attract  the  subscriber's  attention  when  he  is  wanted  at  the 
telephone. 

8. .The  Hook  Switch. — The  hook  switch  is  used  to  connect 
or  disconnect  the  receiver  and  transmitter  from  the  line.  When 
the  instrument  is  not  in  use  the  receiver  hangs  on  the  hook,  hold- 
ing it  down,  connecting  the  ringer  to  the  line  so  that  the  bell  can 
be  rung  when  the  subscriber  is  wanted.  While  the  ringer  is 
connected  to  the  line  the  talking  system,  consisting  of  the  trans- 
mitter and  receiver,  is  disconnected  from  the  line.  When  the 
subscriber  answers  his  call  by  removing  the  receiver,  the  hook  is 
raised  by  a  spring  and  the  ringing  system  is  disconnected  from 
the  line,  while  the  talking  circuit  is  connected  to  the  line. 

9.  The  Induction  Coil. — The  induction  coil  is  used  to  increase 
the  distance  speech  can  be  transmitted.     The  induction  coil 
consists  of  an  iron  core  on  which  are  two  separate  windings  of 
insulated  wire.     The  principles  of  operation  are  explained  later. 

10.  The  Battery. — While  it  is  well  known  that  the  operation 
of  the  telephone  depends  upon  electricity,  the  exact  nature  of 
electricity  is  not  known,  although  a  number  of  laws  governing 
its  action  have  been  determined  by  observation  and  experiment. 
In  order  to  understand  the  operation  of  the  telephone,  it  is 
necessary  to  know  something  of  these  laws  of  electricity.     The 
electrical  supply  for  telephone  work  is  derived  from  batteries, 
or  generators. 


INTRODUCTORY  5 

In  Fig.  2  a  local  battery  telephone  is  shown;  that  is,  each  in- 
strument has  its  individual  battery.  In  large  systems  the 
central  battery  is  used,  or,  in  other  words,  a  single  battery  in 
the  central  office  supplies  current  for  all  the  instruments  in 
use. 


CHAPTER  II 


ELEMENTARY  ELECTRICAL  PRINCIPLES 

Electric  batteries  may  be  divided  into  two  general  classes  and 
are  known  as  primary  and  secondary  batteries.  A  primary 
battery  is  a  device  used  to  generate  an  electrical  pressure  by 
means  of  chemical  action;  that  is,  the  chemical  energy  of  the 
battery  is  changed  into  electrical  energy.  The  secondary 
battery,  or  as  it  is  more  often  called,  the  storage  battery,  also 
makes  use  of  chemical  action  in  supplying  electrical  energy.  But 
before  such  a  battery  can  give  off  electricity,  it  must  be  charged. 
That  is,  electricity  must  be  passed  through 
it.  Primary  batteries  are  used  in  local  bat- 
tery telephone  systems,  and  in  the  central 
battery  system  storage  batteries  are  used. 

11.  Primary  Batteries. — A  simple  primary 
cell  or  battery  may  be  made  by  placing  a 
strip  of  amalgamated  zinc1  and  one  of  copper 
in  a  glass  partly  filled  with  a  mixture  of 
sulphuric  acid  and  water,  care  being  taken 
that  the  metals  do  not  touch  each  other, 
see  Fig.  3.  As  far  as  can  be  seen  no  chemical 
action  is  taking  place  with  the  battery  as 
shown.  However,  if  the  two  plates  be  con- 
nected by  a  wire,  it  may  be  noticed  that 
the  zinc  plate  is  being  consumed  and  that  bubbles  of  gas  are 
formed  on  the  surface  of  the  copper  plate.  If  the  wire  con- 
nection be  broken,  the  chemical  action  ceases.  It  is  evident 
that  there  is  some  action  going  on  when  the  plates  are  con- 
nected which  does  not  take  place  when  the  connection  is  broken. 
The  fact  is  that  an  electrical  current  is  flowing  in  the  wire. 

The  plates  of  a  battery  are  known  as  the  elements  or  electrodes, 
and  the  solution  in  which  they  are  placed  is  known  as  the  electro- 

1  A  piece  of  zinc  may  be  amalgamated  by  cleaning  it  with  diluted  sulphuric 
acid  and  then  rubbing  the  surface  with  mercury.  One  part  of  sulphuric 
acid  poured  into  twenty  parts  of  water  makes  a  mixture  of  the  proper 
strength. 

6 


FIG.  3. 


GLASS    jAff 


SAL. 

AMMONIAC 

SOLUTION 


ELEMENTARY  ELECTRICAL  PRINCIPLES  7 

lyte.  In  the  above-mentioned  battery  the  zinc  and  copper  plates 
are  the  elements  or  electrodes,  and  the  sulphuric  acid  is  the 
electrolyte.  In  order  to  have  an  electrical  action  it  is  not 
necessary  to  have  plates  of  copper  and  zinc  and  an  electrolyte 
of  sulphuric  acid,  for  many  other  substances  may  be  used  in 
batteries. 

One  of  the  most  common  types  of  commercial  batteries  has 
elements  of  carbon  and  zinc  and  an  electrolyte  of  sal  ammoniac. 
The  arrangement  of  such  a  battery  is  shown  in  Fig.  4.  This 
type  of  battery  is  known  as  the  Le 
Clanche  cell.  Dry  cells  are  modified 
Le  Clanche  cells. 

12.  Electrical    Pressure.— The     two 
plates  of  a  battery  are  said  to  be  charged 
if  an  electrical  current  flows  from  one 
to  the  other  when  they  are  connected 
by  a  wire.     An  electrical  pressure,  which 
causes  the  electricity  to  move  from  one 
point  to   another,    exists    between    the 
two  charged  plates.     One  of  the  plates 
of  a  battery  is  said  to  be  charged  posi- 
tively (+)  and  the  other  is  said  to  be  charged  negatively  (  — ). 

Electricity  behaves  in  many  respects  like  water,  and  it  is 
just  as  necessary  to  have  a  difference  in  pressure  between  the 
two  ends  of  an  electrical  conductor  if  we  are  to  have  a  current 
flow  as  it  is  to  have  a  difference  in  pressure  between  the  two 
ends  of  a  water  pipe  if  we  are  to  have  the  water  run  through  the 
pipe.  It  is  easy  to  see  from  the  above  statements  that  if  two 
points  having  equal  electrical  pressures  are  connected,  no 
electricity  will  move  from  one  point  to  the  other. 

As  water  flows  from  a  point  of  high  pressure  to  one  of  lower 
pressure,  so  an  electrical  current  flows  from  a  point  of  high 
pressure  or  potential,  as  it  is  usually  called,  to  one  of  lower 
potential.  It  is  customary,  in  speaking  of  two  electrical  charges, 
to  speak  of  the  charge  having  the  high  pressure  as  positive  (+) 
and  the  one  of  lower  pressure  as  negative  (  — ).  Accordingly 
an  electric  current  flows  from  a  positive  to  a  negative  point. 
In  the  wire  connecting  the  electrodes  of  the  Le  Clanche  cell  the 
current  flows  from  the  carbon  (+)  to  the  zinc  (  — ). 

13.  Telephone  Batteries. — In  early  telephone  practice  some 
form  of  Le  Clanche  cell,  similar  to  Fig.  4,  was  largely  used. 


8 


PRINCIPLES  OF  THE  TELEPHONE 


This  battery  costs  little  to  operate,  as  the  materials  used  in  its 
construction  are  not  expensive,  and  when  the  battery  is  idle  there 
is  no  waste  as  the  sal  ammoniac  does  not  attack  the  zinc  to  any 
extent  except  when  current  is  flowing.  Such  a  cell  requires  little 
attention  except  to  replace  the  water  lost  by  evaporation,  and 
to  replace  the  zinc  element  when  it  has  been  destroyed. 

At  present  a  later  development  of  the  Le  Clanche  battery, 
known  as  a  dry  cell,  is  used  almost  to  the  exclusion  of  other  forms 
of  primary  cells  in  telephone  operation. 

The  dry  cell  has  electrodes  of  carbon  and  zinc,  the  zinc  being 
in  the  form  of  a  cylindrical  cup  and,  in  addition  to  being  one  of 
the  battery  elements,  it  also  acts  as  a  container  for  the  electro- 
lyte. One  form  of  dry  cell  is  shown  in 
Fig.  5.  The  carbon  element  is  in  the  form 
of  a  rod,  and  is  held  in  the  center  of  the  zinc 
cup,  without  touching  it  at  any  point,  the 
electrolyte  occupying  the  intervening  space. 
The  electrolyte,  instead  of  being  a  liquid  as 
in  the  wet  batteries,  is  in  the  form  of  a 
paste  consisting  of  sal  ammoniac,  manga- 
nese dioxide,  carbon,  and  water,  in  varying 
proportions  depending  upon  the  make  of 
the  cell.  Several  thicknesses  of  blotting 
paper  separate  the  electrolyte  from  the  zinc 
cup  so  that  only  the  sal  ammoniac  which  is 
dissolved  in  the  water  can  come  into  contact 
with  it.  The  outside  of  the  cell  is  protected  by  a  pasteboard 
covering  which  also  insulates  the  cell  from  other  cells  of  the 
same  battery  in  case  more  than  one  are  used.  The  cell  is  sealed 
with  pitch  or  wax.  The  standard  size  of  dry  cell  for  telephone 
work  is  2%  in.  in  diameter  and  6  in.  high. 

Dry  cells  have  replaced  wet  batteries  in  telephone  work 
on  account  of  their  smaller  size,  the  fact  that  they  are  not 
easily  broken,  can  not  be  spilled,  have  a  low  first  cost,  and 
require  no  attention  except  renewal  when  they  are  worn  out. 

14.  Conductors  and  Insulators. — When  the  two  plates  of  the 
simple  battery  mentioned  above  are  not  connected  there  is 
no  current  flow  from  one  to  the  other,  as  shown  by  the  fact 
that  there  is  no  chemical  action  taking  place;  neither  is  there 
any  action  when  the  plates  are  connected  by  pieces  of  wood, 
rubber,  or  glass.  If,  however,  a  wire  of  iron,  copper,  or  other 


FIG.  5. 


ELEMENTARY  ELECTRICAL  PRINCIPLES  9 

metal  join  the  plates,  a  current  flows  from  one  plate  to  the 
other.  In  other  words,  the  air,  wood,  glass,  etc.,  do  not  conduct 
the  electricity,  while  the  wires  do.  All  metals  or  substances 
which  conduct  electricity  readily  are  known  as  conductors. 
Substances  which  do  not  conduct  electricity  readily,  such  as 
rubber,  glass,  etc.,  are  known  as  nonconductors  or  insulators. 
Any  insulator  will  conduct  some  little  electricity,  however  slight 
that  quantity  may  be;  and  every  conductor  will  offer  some 
resistance  to  the  flow  of  electricity. 

15.  Electrical  Resistance. — Electrical  resistance  is  the  name 
given  to  that  property  of  a  conductor  which  resists  or  opposes 
the  passage  of  electricity  through  it.     Electrical  resistance  may 
be  compared  with  the  resistance  which  a  stream  of  water  en- 
counters in  flowing  through  a  pipe,  the  pressure  in  one  case  forc- 
ing the  water  to  flow,  and  in  the  other  case  an  electrical  pressure 
causing  the  electricity  to  move  from  one  point  to  another.     The 
electrical  resistance  of  a  conductor  depends  upon  the  material 
of  which  the  conductor  is  made,  and  upon  the  size  and  length  of 
the  conductor. 

16.  Resistivity. — As    some    substances    conduct    heat    more 
readily  than  others,  so  in  the  case  of  electricity  some  substances 
conduct  it  more  readily  than  others. 

TABLE  I 


Metal 

Resistivity  in  ohms  at  0°C. 

Silver,  annealed  

8.781 

Silver,  hard-drawn  
Copper,  annealed               .      .          

9.538 
9  61 

Copper,  hard-drawn 

9  86 

Aluminum   annealed 

15  8 

Aluminum,  hard-drawn  

15.93 

Platinum  
Iron                                                              .  . 

54.35 
58  31 

Tin 

79  29 

Lead 

115  1 

Mercury          

565.9 

In  order  to  compare  the  conductivities  of  different  substances 
some  unit  of  conductor  must  be  agreed  upon.  In  scientific 
calculations  the  unit  conductor  is  one  whose  length  is  1  cm.  and 
whose  cross -sectional  area  is  1  sq.  cm.  In  practical  work, 


10  PRINCIPLES  OF  THE  TELEPHONE 

however,  the  unit  conductor  is  a  piece  of  circular  wire  1  ft. 
long  and  Kooo  (0.001)  in.  in  diameter.  The  resistance  of 
such  a  conductor  is  called  the  resistivity  of  the  material  of  which 
the  wire  is  made. 

Table  I  gives  the  resistivities  of  a  number  of  metals.  The 
first,  silver,  has  the  lowest  resistivity,  while  the  last,  mercury, 
has  the  highest  resistivity  of  the  metals  given. 

If  we  wish  merely  to  compare  the  resistivities  of  wires  of 
the  same  size  and  lengths  but  of  different  materials  we  can  call 
the  resistivity  of  silver  unity.  The  relative  resistivities  are  then 
as  follows: 

TABLE  II 


Metal 

Relative  resistivity 

Silver 

1  0 

Copper.  . 

1.09 

Aluminum  
Platinum 

1.8 
6  19 

Iron  

6.04 

Tin  

9  03 

Lead 

13  1 

Mercury  

64.4 

From  the  table  it  can  be  seen  readily  why  so  much  copper 
wire  is  used  in  electrical  apparatus.  Silver,  which  is  a  better 
conductor  and  would  be  more  desirable  for  some  work  for  that 
reason,  costs  several  times  as  much  as  copper. 

The  resistance  of  a  conductor  depends  upon  its  length.  The 
longer  the  conductor,  the  greater  its  resistance.  Thus  a  No. 
16  wire  200  ft.  long  will  have  twice  as  great  a  resistance  as  100 
ft.  of  the  same  wire. 

A  small  conductor  offers  a  greater  resistance  to  the  flow  of 
an  electric  current  than  does  a  large  one  of  the  same  material, 
just  as  a  small  pipe  hinders  the  flow  of  water  more  than  a  large 
one.  The  resistance  of  conductors  varies  inversely  as  the 
areas  of  their  cross-sections,  or  in  other  words,  inversely  as  the 
quantity  of  material  in  a  given  length. 

17.  Wire  Measurement. — In  this  country  the  length  of  wire 
is  usually  given  in  feet,  and  the  size  is  specified  either  by  diameter, 
cross-sectional  area,  or  gage  number.  The  units  used  for  the 
measurement  of  the  diameter  and  cross-sectional  area  are  not 
the  inch  and  square  inch,  but  the  mil  and  circular  mil. 


ELEMENTARY  ELECTRICAL  PRINCIPLES         11 

The  Mil.  —  The  unit  of  length  in  measuring  the  diameter  is  the 
Kooo  (=  0.001)  in.  and  is  called  the  mil.  A  1-in.  cable  has  a 
diameter  of  1,000  mils.  The  diameter  of  a  wire  0.25  in.  is  equal 
to  250  mils,  etc. 

Circular  Mils.  —  A  circle  whose  diameter  is  0.001  in.  (=1 
mil)  is  said  to  have  an  area  of  1  cir.  mil.  Since  the  areas  of  two 
circles  having  different  diameters  are  to  each  other  as  the  squares 
of  their  diameters,  to  express  the  cross-section  of  any  wire  in 
circular  mils,  when  its  diameter  in  mils  is  given,  all  that  is 
necessary  is  to  square  the  diameter,  that  is,  multiply  the  diameter 
by  itself. 

EXAMPLES 

1.  What  is  the  cross-sectional  area  in  circular  mils  of  a  wire  %  in.  in 
diameter? 

Solution 

Y±  in.  =  0.25  in. 
0.25  in.  =  25Kooo  =  250  mils 

Area  in  circular  mils  equals  diameter  squared 

Diameter  =  250  mils 

2502  =  250  X  250  =  62,500  cir.  mils. 

2.  A  No.  0000  wire  has  a  cross-sectional  area  of  211,600  cir.  mils.     What 
is  its  diameter  in  mils  and  in  inches? 

Solution 

Since  the  cross-sectional  area  in  circular  mils  is  equal  to  the  square  of 
the  diameter  in  mils,  the  diameter  in  mils  must  be  equal  to  the  square  root 
of  the  cross-sectional  area.  In  symbols 

D2  =  area 
and  D  =  \/area 
But  area  =  211,600  cir.  mils 
Hence  D  =  \/21MH)6 
=  460  mils 

1  mil  =  Kooo  in. 

460 
Then   460   mils  =  =  0.46  in. 


18.  Gage  Numbers.  —  In  the  United  States  practically  the  only 
gage  now  used  for  copper  wire  is  the  American  Wire  Gage  com- 
monly called  the  Brown  and  Sharpe  (B.  &  S.)  Gage.  This  gage 
was  devised  in  1857  by  J.  R.  Brown,  one  of  the  founders  of  the 
Brown  &  Sharpe  Manufacturing  Co.  In  this  gage  the  size  of 
wire  is  specified  by  number.  The  mathematical  law  on  which 


12  PRINCIPLES  OF  THE  TELEPHONE 

TABLE  III. — TABULAR  COMPARISON  OP  WIRE  GAGES,  DIAMETERS  IN  MILS 


Gage  No. 

American  Wire  Gage 
(B.  &  S.) 

Steel  Wire 
Gage 

Birmingham  Wire 
Gage  (Stubs') 

(British)  Standard 
Wire  Gage 

7-0 

490.0 

500.0 

6-0 

461.5 

464.0 

5-0 

430.5 

432.0 

4-0 

460.0 

393.8 

454.0 

400.0 

3-0 

410.0 

362.5 

425.0 

372.0 

2-0 

365.0 

331.0 

380.0 

348.0 

0 

325.0 

306.5 

340.0 

324.0 

1 

289.0 

283.0 

300.0 

300.0 

2 

258.0 

262.5 

284.0 

276.0 

3 

229.0 

243.7 

259.0 

252.0 

4 

204.0 

225.3 

238.0 

232.0 

5 

182.0 

207.0 

220.0 

212.0 

6 

162.0 

192.0 

203.0 

192.0 

7 

144.0 

177.0 

180.0 

176.0 

8 

128.0 

162.0 

165.0 

160.0 

9 

114.0 

148.3 

148.0 

144.0 

10 

102.0 

135.0 

134.0 

128.0 

11 

91.0 

120.5 

120.0 

116.0 

12 

81.0 

105.5 

109.0 

'     104.0 

13 

72.0 

91.5 

95.0 

92.0 

14 

64.0 

80.0 

83.0 

80.0 

15 

57.0 

72.0 

72.0 

72.0 

16 

51.0 

62.5 

65.0 

64.0 

17 

45.0 

54.0 

58.0 

56.0 

18 

40.0 

47.5 

49.0 

48.0 

19 

36.0 

41.0 

42.0 

.   40.0 

20 

32.0 

34.8 

35.0 

36.0 

21 

28.5 

31.7 

32.0 

32.0 

22 

25.3 

28.6 

28.0 

28.0 

23 

22.6 

25.8 

25.0 

24.0 

24 

20.1 

23.0 

22.0 

22.0 

25 

17.9 

20.4 

20.0 

20.0 

26 

15.9 

18.1 

18.0 

18.0 

27 

14.2 

17.3 

16.0 

16.4 

28 

12.6 

16.2 

14.0 

14.8 

29 

11.3 

15.0 

13.0 

13.6 

30 

10.0 

14.0 

12.0 

12.4 

31 

8.9 

13.2 

10.0 

11.6 

32 

8.0 

12.8 

9.0 

10.8 

33 

7.1 

11.8 

8.0 

10.0 

34 

6.3 

10.4 

7.0 

9.2 

35 

5.6 

9.5 

5.0 

8.4 

36 

5.0 

9.0 

4.0 

7.6 

37 

4.5 

8.5 

6.8 

38 

4.0 

8.0 

6.0 

39 

3.5 

7.5 

5.2 

40 

3.1 

7.0 

4.8 

ELEMENTARY  ELECTRICAL  PRINCIPLES         13 

this  gage  is  based  is,  the  ratio  of  any  diameter  to  the  next  smaller 
is  a  constant  number. 

For  practical  purposes  tables  are  prepared  giving  the  gage 
number,  diameter  in  mils  or  inches,  cross-sectional  area  in  cir- 
cular mils,  and  other  data  that  may  be  useful,  depending  upon 
the  completeness  of  the  table.  The  numbers  usually  range 
from  0000  to  40.  The  diameter  of  No.  0000  is  460  mils  and  of 
No.  40,  3.145  mils.  The  student  will  thus  see  that  the  larger  the 
gage  number  the  smaller  the  diameter.  A  wire  table  for  ordinary 
practical  calculations  is  given  on  page  14.  This  table  was 
prepared  by  the  Bureau  of  Standards  and  is  published  in  circular 
No.  31  together  with  others  of  greater  accuracy  and  detail. 

The  telephone  companies  still  use  the  "  Standard  Wire  Gage," 
otherwise  known  as  the  New  British  Standard  (N.B.S.)  Gage, 
for  copper  wire  and  the  Birmingham  Gage  for  steel  wire.  The 
other  steel  wire  gage  used  in  this  country  is  now  known  as  the 
Steel  Wire  Gage  (Stl.W.G.).  Manufacturers  of  wire  as  a  rule 
prefer  that  the  size  of  wire  be  specified  in  decimal  fractions  of  an 
inch,  without  the  use  of  gage  numbers.  A  comparative  table  of 
the  four  gages  used  by  telephone  companies  is  given  in  Table  III. 

19.  Unit  of  Resistance. — In  order  that  the  resistance  of  dif- 
ferent wires  may  be  compared,  a  unit  of  resistance  has  been 
adopted.     This  unit  of  resistance  is  known  as  the  ohm.     The 
resistance  of  1,000  ft.  of  No.  10  copper  wire  at  a  temperature  of 
20°C.  or  68°F.  is  about  1  ohm. 

The  standard  ohm  is  defined  as  the  resistance  offered  to  the 
flow  of  an  unvarying  electric  current  by  a  column  of  mercury 
106.3  cm.  long,  14.4521  grams  mass,  and  of  a  constant  cross- 
sectional  area,  at  a  temperature  of  melting  ice. 

20.  Unit  of  Electrical  Pressure. — Just  as  it  has  been  neces- 
sary to  choose  a  unit  of  resistance,  so  it  has  been  necessary  to 
choose  a  unit  of  electrical  pressure  or  electromotive  force,  as 
this  pressure  is  usually  called.     The  unit  of  pressure  is  known 
as  the  volt.     Electromotive  force  or  electrical  pressure  is  not 
electricity;  it  is  merely  the  force  that  causes  electricity  to  move, 
so  that  the  voltage  of  a  battery  is  no  measure  of  the  quantity 
of  electricity  that  can  be  obtained  from  that  battery,  but  is 
merely  a  measure  of  the  electrical  pressure  existing  between  the 
two  plates.     The  pressure  in  a  standard  dry  cell  is  about  1J^ 
volts.     A  voltmeter  is  an  instrument  for  measuring  electrical 
pressure.     Since  the  electrical  pressure  of  a  battery  exists  be- 


14 


PRINCIPLES  OF  THE  TELEPHONE 


tween  'its  two  electrodes,  in  order  to  secure  a  voltmeter  read- 
ing of  this  pressure  the  instrument  must  be  connected  to  the 
electrodes. 

TABLE  IV. — NUMBER,   DIMENSIONS,  AND  RESISTANCE  OF  STANDARD  AN- 
NEALED COPPER  WIRE  (SOLID) 


No. 

Diameter 

Area 

Weight 

Resistance  in  ohms  at  68°F. 

A.W.  Gage 

In  mils 

Circular  mils 

Lb.  per  1,000  ft. 

Ohms  per  1,000  ft.i 

6 

162.0 

26,250.0 

79.46 

0.3951 

7 

144.3 

20,820.0 

63.02 

0.4982 

8 

128.5 

16,510.0 

49.98 

0.6282 

9 

114.4 

13,090.0 

39.63 

0.7921 

10 

101.9 

10,380.0 

31.43 

0.9989 

11 

90.74 

8,234.0 

24.92 

1.260 

12 

80.81 

6,530.0 

19.77 

1.588 

13 

71.96 

5,178.0 

15.68 

2.003 

14 

64.08 

4,107.0 

12.43 

2.525 

15 

57.07 

3,257.0 

9.858 

3.184 

16 

50.82 

2,583.0 

7.818 

4.015 

17 

45.26 

2,048  .  0 

6.200 

5.064 

18 

40.30 

1,624.0 

4.917 

6.385 

19 

35.89 

1,288.0 

3.899 

8.051 

20 

31.96 

1,022.0 

3.092 

10.15 

21 

28.46 

810.1 

2.452 

12.80 

22 

25.35 

642.4 

1.945 

16.14 

23 

22.57 

509.5 

1.542 

20.36 

24 

20.10 

404.0 

1.223 

25.67 

25 

17.90 

320.4 

0.9699 

32.37 

26 

15.94 

254.1 

0.7692 

40.82 

27 

14.20 

201.5 

0.6100 

51.46 

28 

12.64 

159.8 

0.4837 

64.90 

29 

11.26 

126.7 

0.3836 

81.84 

30 

10.03 

100.5 

0.3042 

103.2 

31 

8.928 

79.70 

0.2413 

130.1 

32 

7.950 

63.21 

0.1913 

164.1 

33 

7.080 

50.13 

0.1517 

206.9 

34 

6.305 

39.75 

0.1203 

260.9  . 

35 

5.615 

31.52 

0.09542 

329.0 

36 

5.000 

25.00 

0.07568 

414.8 

37 

4.453 

19.83 

0.06001 

523.1 

38 

3.965 

15.72 

0.04759 

659.6 

39 

3.531 

12.47 

0.03774 

831.8 

40 

3.145 

9.888 

0.02993 

1049.0 

1  For  hard-drawn  copper  wire  increase  these  values  by  2.7  per  cent. 

21.  Electric  Current. — In  order  to  have  a  flow  of  electricity 
from  one  point  to  another  it  is  necessary  that  a  difference  in 


ELEMENTARY  ELECTRICAL  PRINCIPLES         15 

electrical  pressure  exist  between  the  two  points,  and  that  these 
points  be  connected  by  a  conductor. 

The  rate  of  flow  in  any  circuit  depends  upon  the  pressure  or 
voltage,  and  the  resistance  of  the  path  through  which  the  cur- 
rent flows.  It  is  evident  that  a  greater  rate  of  flow  will  take 
place  under  a  high  pressure  than  under  a  low  one.  Thus  a 
pressure  of  2  volts  would  cause  twice  as  much  electricity  to  flow 
through  a  circuit  in  a  given  time  as  a  pressure  of  1  volt.  The 
resistance  which  has  to  be  overcome  in  any  conductor  determines 
the  current,  since  the  greater  the  resistance,  the  more  the  current 
will  be  held,  back  or  hindered  in  flowing  through  the  conductor. 

22.  The  Ampere. — The  ampere  is  the  unit  of  current,  and  is 
the  current  that  will  flow  in  a  circuit  having  a  resistance  of  1 
ohm  under  an  electrical  pressure  of  1  volt.     An  ammeter  is  an 
instrument  for  measuring  the  rate  of  flow  of  electricity  in  a 
circuit. 

23.  Pressure,  Current,  and  Resistance. — It  has  been  shown 
above  that  electric  current  depends  upon  the  pressure  and  re- 
sistance of  a  circuit,  or  that  for  different  pressures  and  resistances 
different  currents  will  flow.     Thus  if  a  certain  current  is  flowing 
in  a  circuit  and  it  is  desired  to  change  that  current,  it  is  neces- 
sary to  change  either  the  pressure  or  resistance  of  the  circuit. 

The  units  of  pressure,  resistance,  and  current  have  been  so 
chosen  that  if  two  of  these  quantities  are  known,  the  third  can 
be  easily  calculated.  Since  1  ampere  represents  the  current  in  a 
circuit  having  a  resistance  of  1  ohm  under  a  pressure  of  1  volt,  in 
order  to  have  2  amperes  flow  in  the  same  circuit  it  would  be  neces- 
sary to  have  a  pressure  of  2  volts,  etc.  A  simple  way  of  express- 
ing this  relationship  is:  Current  equals  pressure  divided  by 
resistance,  or 

Volts 
Amperes  =  Ohms 

In  telephone  operation  the  pressure  remains  constant,  so  that 
the  fluctuating  current  which  is  necessary  to  send  a  message  is 
obtained  by  the  rapid  variations  in  the  resistance  of  the  trans- 
mitter when  it  is  being  used. 

24.  Electric  Circuits. — An  electric  circuit  is  a  system  of  con- 
ductors and  apparatus  connected  so  that  a  current,  under  certain 
conditions,  may  flow  from  one  point  to  another  point  on  the  con- 
ductors.    With  reference  to  the  manner  in  which  the  conductors 
are  connected,  we  have  two  general  classes  of  circuits;  namely, 


16 


PRINCIPLES  OF  THE  TELEPHONE 


series  and  parallel.     With  reference  to  the  possibility  of  current 
flowing,  circuits  are  classed  as  closed  and  open. 

25.  Series  Circuit. — A  series  circuit  is  one  in  which  the  current 
must  flow  through  each  part  of  the  circuit  in  succession.     The 
current  has  the  same  strength  at  whatever  point  in  the  circuit 
it  is  measured.     Fig.  6a  is  a  series  circuit  consisting  of  a  magneto 
generator,  two  wires  and  a  telephone  ringer. 

26.  Parallel  Circuit. — Parallel  circuits  are  shown  in  Fig.  6&. 
The  current  from  the  generator  divides  and  goes  through  the 
bell  coils  in  parallel.     Thus  a  parallel  circuit  is  one  consisting 


FIG.  6a. 


FIG.  6&. 


FIG.  6c. 


FIG.  6d 


of  two  or  more  individual  circuits  connected  in  parallel,  or,  in 
short,  a  parallel  circuit  is  a  divided  circuit. 

27.  Closed  Circuit. — When  the  conductors  are  connected  so 
that  a  current  can  flow,  the  circuit  is  said  to  be  closed.     Fig.  6a 
is  also  a  closed  series  circuit. 

28.  Open  Circuit. — An  open  electrical  circuit  is  one  in  which 
some  part  is  disconnected  so  that  a  current  can  not  flow.     Fig. 
6c  shows  an  open  circuit,  opened  at  the  jack,  and  at  the  magneto. 

29.  Short  Circuit. — A  short  circuit  is  said   to   exist   when   a 
shunt  or  parallel  circuit  of  comparatively  low  resistance  has  been 
connected  to  the  main  circuit.     A  shunt  is  one  of  the  branches 
of  a  parallel  circuit.     Fig.  6d  shows  a  connection  from  the  point 
a  to  the  point  b  short-circuiting  the  generator. 


ELEMENTARY  ELECTRICAL  PRINCIPLES 


17 


30.  Grounded  Circuit. — A  circuit  is  said  to  be  grounded  when 
any  part  of  it  is  connected  to  the  ground.  Such  a  circuit  is 
shown  in  Fig.  6e.  In  many  telephone  systems  the  ground  is 
made  a  part  of  the  circuit,  but  one  wire  being  used.  This 
is  made  possible  by  the  fact  that  wet  earth  is  a  good  conductor 
of  electricity. 


FIG.  6e. 

31.  Resistance  of  a  Series  Circuit. — When  conductors  are 
connected  end  to  end  so  that  the  total  current  must  flow  through 
each  conductor  in  succession,  the  joint  resistance  of  the  con- 
ductors or  the  resistance  of  the  circuit  is  the  sum  of  the  resist- 
ances of  the  individual  conductors  so  connected.  Thus  in  Fig. 
7  the  resistance  of  the  receiver  circuit  is  70  +  10  +  87.5  +  175 
+  10  +  32  =  384.5  ohms. 

IO    OHMS 


10  OHMS 


FIG.  7. 


32.  Resistance  of  a  Parallel  Circuit. — When  several  conductors 
are  connected  in  parallel,  as  the  ringers  in  Fig.  66,  the  joint  con- 
ductivity is  the  sum  of  the  conductivities  of  the  several  branches. 
This  will  be  more  evident  if  we  consider  a  hydraulic  analogy. 
Suppose  two  large  water  mains  are  connected  by  several  small 


18  PRINCIPLES  OF  THE  TELEPHONE 

pipes.  It  is  very  evident  that  the  current  of  water  flowing  from 
one  main  to  the  other  main  is  equal  to  the  sum  of  the  currents 
in  the  small  pipes.  That  is,  the  joint  conductivity  is  equal  to 
the  sum  of  the  conductivities  of  the  small  pipes.  In  exactly  an 
analogous  manner,  when  the  circuit  consists  of  several  conductors 
in  parallel,  the  total  current  is  the  sum  of  the  currents  in  the 
several  parallel  conductors.  This  fact,  together  with  Ohm's 
law,  enables  us  to  calculate  the  joint  resistance  in  the  following 
manner : 

Suppose  the  resistances  of  the  ringers  in  Fig.  66  are  R^  R2, 
and  Rz  respectively,  and  that  a  difference  of  electrical  pressure 
of  E  volts  exists  between  their  terminals;  then  by  Ohm's  law  the 
current  in  ringer  1  is 

E 
l^R, 

E 

current  in  2  is  iz  =  -5- 
Kz 

and 

E 

current  in  3  is  is  =  -5- 
riz 

If  R  is  the  joint  resistance,  the  total  current  is 

7-^ 
"  R 

But  ii  +  i*  +  *s  =  I  ] 
Therefore 

E]=  E_       E_       E_ 

R       R\       RZ       Rs 
or 

1=1+1+1 

R       RI       RZ       RS 

Solving  this  for  R,  we  get 

RiXRzX  Rs 


Ri  X  R*  +  Ri  X  Rz  +  #2  X  fla 

If  the  resistances  of  the  bells  are  equal  then 
R\  =  RZ  —  Rs,  and 


That  is,  the  joint  resistance  is  equal  to  one-third  the  resistance 
of  one  bell. 


ELEMENTARY  ELECTRICAL  PRINCIPLES         19 

EXAMPLE 

Three  ringers  whose  resistances  are  1,000  ohms,  1,600  ohms,  and  2,000 
ohms  are  bridged  across  a  line  to  which  is  connected  a  storage  battery  of 
24  volts. 

(a)  What  is  the  current  in  each  bell? 

(6)  What  is  the  total  current  given  out  by  the  battery? 

(c)   What  is  the  joint  resistance  of  the  ringers? 

Solution 
(a)  By  Ohm's  law  the  several  currents  are 

*l  =  iloo  =  a°24  amP- 

24 

=  0.015  amp. 


-, 

and 

24 

>•*  =  Poo  =  °-012  amP- 

(6)   The  total  current  is  equal  to  the  sum  of  ii  +  i2  +  i3  or 
7  =  0.024  +  0.015  +  0.012  =  0.051  amp. 

(c)   The  joint  resistance  may  be  calculated  in  two  ways.     First  by  Ohm's 
law 

R      E 
=  -j 

24 

= 

or  by  the  formula 


D 
it  = 


R1XR2XRS  =  1,000  X  1,600  X  2,000 

=  3,200,000,000 
RiXR2=  1,600,000 
R!  X  Ra  =  2,000,000 
R2  x  Rz  =  3,200,000 


#1  X  R*  +  Ri  X  Rs  +  #2  X  #3  =  6,800,000 
and 

_       32,000 
~68~ 

_  32,000       8,000 
68  17 

=  470.6  ohms,  nearly. 

To  calculate  the  joint  resistance  of  any  number  of  conductors  connected 
in  parallel,  we  proceed  in  exactly  the  same  way.  It  is  not  necessary  to  show 
how  any  more  formulas  are  calculated.  A  general  rule  will  suffice. 

Rule. — To  find  the  joint  resistance  of  any  number  of  parallel  conductors, 
3 


20 


PRINCIPLES  OF  THE  TELEPHONE 


divide  the  product  of  the  resistances  of  all  of  the  conductors  by  the  sum  of  the 
products  obtained  by  multiplying  together  all  of  the  resistances  less  one.  The 
same  resistance  must  not  appear  in  any  partial  product  more  than  once. 

33.  Cells  in  Series. — In  the  application  of  Ohm's  law  the 
electromotive  force  E  must  be  the  total  electrical  pressure  in 

the  circuit.  It  is,  therefore,  neces- 
sary to  be  able  to  calculate  the 
pressure  when  cells  are  connected 
in  series  or  in  parallel.  Cells  are 
said  to  be  connected  in  series  when 
G  the  carbon  electrode  of  one  is  con- 


^J^^= 

=^r=/3>= 

^==^=-- 

• 

n  a 

r 

L 

i. 

C 

a 

6 

•  / 
/? 


FIG.  8. 


-M/txz'  c/Che 
FIG.  9. 


nected  to  the  zinc  electrode  of  the  next  and  so  on  as  shown  in 
Fig.  8. 

The  analogous  diagram  of  tanks  in  series  may  help  to  show 
how  the  total  pressure  is  calculated.  The  hydrostatic  pressure 
at  A  is  evidently  the  sum  of  the  pressures  due  to  the  elevations 
of  the  water  AB  +  BC  +  CD;  that  is,  the  sum  of  the  pressures 
in  the  individual  tanks.  Similarly,  the  electrical  pressure  be- 
tween the  terminals  1  and  2  is  the  sum  of  the  pressures  across  the 
cells  a,  6,  and  c.  In  general,  if  E  is  the  pressure  of  one  cell 
and  n  cells  are  connected  in  series,  the  total  pressure  is  nE. 

34.  Cells  in  Parallel. — Fig.  9  is  a  diagram  of  tanks  and  cells 
connected  in  parallel.  It  is  evident  that  the  hydrostatic  pres- 


ELEMENTARY  ELECTRICAL  PRINCIPLES         21 

sure  exerted  by  the  water  in  tank  A  is  the  same  as  that  in  B 
and  C,  since  the  height  of  the  water  is  the  same  in  each.  The 
total  pressure  is  equal  to  that  of  one  tank.  The  three  tanks 
could  be  replaced  by  one  large  tank,  and  as  long  as  the  water 
was  maintained  at  the  same  height,  the  pressure  at  the  orifice 
would  be  exactly  the  same  in  the  two  cases. 

When  cells  are  connected  in  parallel  the  total  pressure  is 
equal  to  the  pressure  of  one  cell,  and  the  three  cells  a,  6,  and 
c,  can  be  replaced  by  one  large  cell  having  the  same  cross-section 
of  zinc  and  carbon  as  the  three  cells  taken  together. 

When  tanks  are  connected  in  parallel  it  is  evident  that  each 
supplies  only  a  part  of  the  current.  The  same  principle  holds 
with  reference  to  cells  connected  in  parallel — each  cell  supplies 
only  a  part  of  the  total  current.  The  student  can  readily  verify 
the  law  of  pressures  by  connecting  three  cells  in  parallel  and 
then  connecting  a  voltmeter  to  terminals  1  and  2,  Fig.  9, 
and  comparing  the  voltmeter  reading  with  the  reading  given  when 
the  voltmeter  is  connected  to  each  cell  separately. 

35.  Battery  Resistance  for  Parallel  Connections. — The  effect 
of  connecting  cells  in  parallel  is  to  increase  the  current  capacity 
and  decrease  the  internal  resistance.  In  so  far  as  the  internal 
resistance  of  one  cell  is  concerned,  it  may  be  considered  as  a 
conductor  whose  resistance  is  r.  Three  cells  in  parallel  will  thus 
be  the  equivalent  of  three  resistances  in  parallel.  It  has  been 
shown  that  when  three  equal  resistances  are  in  parallel,  the 
joint  resistance  is  equal  to  one-third  of  the  resistance  of  one 
wire.  Accordingly,  the  joint  internal  resistance  of  a  battery 


of  m  parallel  cells  is  — 
ra 


EXAMPLE 


Five  cells  each  having  an  internal  resistance  of  1  ohm  are  connected  in 
parallel.     What  is  the  joint  resistance? 

Solution 

Since  the  resistances  of  the  cells  are  the  same,  the  joint  resistance  is 
K  of  1  ohm  =  0.2  ohm. 

QUESTIONS 

1.  Explain  the  steps  necessary  in  order  to  send  a  telephone  message  from 
one  point  to  another. 

2.  Name  the  main  parts  of  a  telephone  instrument  and  give  briefly  the 
uses  of  each. 


22  PRINCIPLES  OF  THE  TELEPHONE 

3.  Of  what  use  is  a  battery  in  telephone  work? 

4.  Explain  how  a  simple  battery  is  made.  . 

5.  What  is  a  conductor?     An  insulator?     Name  the  five  best  conductors 
of  which  you  know.     The  five  best  insulators. 

6.  What  are  the  elements  of  a  battery?     What  is  the  electrolyte? 

7.  What  is  meant  when  an  object  is  said  to  be  "charged?" 

8.  What  do  you  understand  by  electrical  pressure?     Compare  it  with 
water  pressure. 

9.  What  are  the  elements  and  electrolyte  of  the  Le  Clanche  cell?     Explain 
briefly  how  a  dry  cell  is  made. 

10.  What  is  electrical  resistance?     Upon  what  does  the  resistance  of  a 
conductor  depend? 

11.  Which  has  the  greater  resistance,  copper  or  silver?     Iron  or  copper? 
Iron  or  lead? 

12.  Why  is  a  unit  of  resistance  necessary?     What  is  this  unit? 

13.  What  is  the  unit  of  electrical  pressure?     By  what  other  name  is 
electrical  pressure  usually  known?     What  is  the  voltage  of  a  dry  cell? 

14.  What  causes  an  electrical  current  to  flow?     What  conditions  are 
necessary  in  order  to  have  a  current  of  electricity? 

15.  What  is  the  unit  of  current?     Define  it  in  terms  of  volts  and  ohms. 

16.  What  is  the  relation  between  volts,  amperes,  and  ohms? 

17.  What  is  the  rate  of  current  flow  through  a  coil  of  telephone  wire 
having  a  resistance  of  1  ohm,  if  the  ends  are  connected  to  the  terminals  of  a 
dry  cell?     If  another  coil  of  half  the  length  is  used,  what  will  be  the  current 
in  amperes? 

18.  Define  the  following:  Circuit,  open  circuit,  closed  circuit,  short  cir- 
cuit, series  circuit,  parallel  circuit,  shunt. 

19.  Four  resistances  of  80,  95,  40,  and  50  ohms  are  connected  in  series. 
What  is  the  joint  resistance? 

20.  Four  series  ringers  of  80  ohms  resistance  each  were  connected  in 
parallel.     What  was  the  joint  resistance? 

21.  Four  dry  cells,  each  having  an  electromotive  force  of  1.4  volts  and 
an  internal  resistance  of  1  ohm,  were  connected  in  series  to  a  circuit  whose 
resistance  was  10  ohms.     What  was  the  current  in  the  circuit? 

22.  Suppose  the  cells  mentioned  in  question  21  were  connected  in  parallel 
to  the  same  circuit,  what  current  would  flow? 


CHAPTER  III 


BINOING    POSTS , 


MAGNETIC  PRINCIPLES 

36.  Receiver  Action. — An  examination  of  a  telephone  receiver 
shows  the  working  parts  to  consist  of  a  permanent  magnet  on 
which  coils  of  fine  insulated  wire  are  wound,  and  a  thin  iron 
disk  mounted  close  to,  but  not  touching,  the  poles  of  the  magnet. 
This  arrangement  is  shown  in  Fig.  10  for  a 

single-pole  receiver,  which  was  the  earliest 
type  in  use. 

This  examination  also  shows  the  iron  disk 
or  diaphragm  to  be  attracted  by  the  per- 
manent magnet.  Since  the  outer  edge  of 
the  disk  can  not  move,  the  disk  will  become 
slightly  "dished,"  as  the  center  is  drawn  in 
toward  the  magnet.  When  the  receiver  is 
not  in  use  this  pull  will  be  steady,  and  there 
will  be  no  movement  of  the  disk.  If  the 
strength  of  the  magnet  be  increased,  how- 
ever, it  will  exert  greater  attraction  for  the 
disk,  and  the  latter  will  be  pulled  closer  to 
the  magnet  pole.  On  the  other  hand,  if  the 
force  of  the  magnet's  attraction  be  decreased 
the  diaphragm  will  spring  away  from  the 
pole,  and  return  more  nearly  to  its  original  shape. 

When  the  receiver  is  in  use,  the  strength  of  the  magnet  is 
changed  from  time  to  time  by  the  fluctuating  line  current  which 
flows  through  the  coil  of  the  receiver.  If  these  changes  in  the 
force  of  the  magnet  take  place  rapidly  enough,  the  disk  will 
vibrate  at  such  a  rate  that  sounds  will  be  produced  by  it.  In 
order  to  understand  how  an  electric  current  flowing  in  the  coil 
can  change  the  strength  of  the  magnet,  it  will  be  necessary  to 
investigate  a  few  of  the  relations  existing  between  electricity 
and  magnetism. 

37.  Magnetism.- — The  magnet,  as  first  known,  existed  in  the 
form  of  a  certain  iron  ore  known  as  magnetite  (so  named  in 

23 


DIAPHRAGM' 

FIG.  10. 


24  PRINCIPLES  OF  THE  TELEPHONE 

honor  of  the  city  of  Magnesia,  where  the  ore  having  this  pecu- 
liarity was  discovered)  which  has  the  property  of  attracting 
pieces  of  iron.  The  strange  force  by  which  the  particles  of  iron 
were  attracted  was  likewise  known  as  magnetism.  These  first 
magnets  were  natural  magnets. 

It  was  found,  somewhat  later,  that  artificial  magnets  could 
be  formed  by  subjecting  pieces  of  iron  to  the  influence  of  a 
magnetizing  force.  One  of  the  early  methods  of  producing  such 
artificial  magnets,  was  by  stroking  or  rubbing  a  piece  of  iron  with 
a  piece  of  magnetic  ore  or  natural  magnet.  There  are  at  present 
other  methods  of  producing  artificial  magnets.  The  first  arti- 
ficial magnets  were  in  the  form  of  a  bar  as  shown  in  Fig.  lla. 


FIG.  lla.  FIG.  lib. 

38.  Magnetic   Substances. — Iron  in  its  various   commercial 
forms,  such  as  wrought  iron,  cast  iron,  steel,  etc.,  is  strongly 
magnetic  and  is  known  as  a  magnetic  substance.     Substances 
such  as  wood,  glass,  copper,  etc.,  which  can  not  be  made  to 
act  as  magnets,  are  known  as  nonmagnetic  substances. 

39.  Magnetic    Induction. — When    a    magnetic    substance    is 
magnetized  by  coming  into  contact  with  a  magnet,  the  substance 
is  said  to  have  been  magnetized  by  induction,  or  the  magnetism 
is  said  to  be  induced  in  the  substance. 

40.  Experiment  1. — Apparatus: 

Bar  Magnet. 
Iron  Filings. 
Wire  Nails. 

(a)  Dip  the  ends  of  the  bar  magnet  into  the  iron  filings, 
and  note  that  the  filings  cling  to  the  ends  of  the  magnet.  The 
parts  to  which  filings  cling  are  called  the  poles  of  the  magnet. 
Those  points  to  which  no  filings  cling  are  known  as  neutral 
points. 

(6)  Rub  a  knife  blade  or  other  piece  of  steel  with  one  end 
of  the  magnet;  always  move  the  magnet  in  the  same  direction 
along  the  knife.  Dip  the  end  of  the  blade  in  the  filings.  Has 
the  blade  become  magnetized? 

(c)  Hold  one  end  of  the  bar  magnet  against  the  head  of  a 


MAGNETIC  PRINCIPLES  25 

nail  and  dip  the  point  of  the  nail  into  the  iron  filings.  Note 
that  a  magnet  pole  has  been  developed  on  the  point  of  the  nail. 
(d)  Try  the  last  experiment  with  a  piece  of  wood  or  short 
piece  of  copper  wire  in  place  of  a  nail,  and  note  that  there  is  no 
evidence  of  these  substances  becoming  magnetized. 

41.  Magnetic  Action. — If  a  bar  magnet  be  suspended  by  a 
fine  thread  attached  to  its  center,  the  magnet  will  turn  so  that 
one  end  will  point  in  a  northerly  direction  and  the  other  in  a 
southerly  direction,  no  matter  what  the  original  position  of  the 
magnet  may  be.     The  end  of  the  magnet  that  will  point  toward 
the  north  is  called  the  North  pole  (marked  N.),  and  the  other  end 
is  called  the  South  pole  (marked  S.).     A  compass  needle  is  merely 
a  very  light  bar  magnet. 

42.  Experiment  2. — Apparatus: 

Horseshoe  Magnet. 
•  Two  Bar  Magnets. 

(a)  Suspend  a  bar  magnet  by  a  fine  thread  attached  to  its 
center.     When  the  magnet  has  come  to  rest,  it  will  point  north 
and  south.     Mark  the  end  that  points  north,  to  indicate  the 
N.   pole.     Suspend  the  second  bar  magnet  in  the  same  way 
and  mark  its  N.  pole.     Bring  the  N.  pole  of  the  first  bar  magnet 
near  the  N.  pole  of  the  suspended  one.     Observe  that  the  two 
poles  repel  each  other.     Now  bring  a  N.  and  S.  pole  near  each 
other  and  observe  the  strong  attraction  exerted  between  the 
two. 

(b)  Using  the  suspended  bar  magnet  as  in  the  first  part  of 
the  experiment,  test  the  poles  of  the  horseshoe  magnet,  and 
mark  the  N.  pole. 

43.  Laws  of  Magnetic  Attraction  and  Repulsion. — If  the  N. 
pole  of  one  bar  magnet  is  brought  near  the  S.  pole  of  the  other, 
a  strong  attraction  is  exerted  between  the  two;  but  if  the  two  N. 
or  two  S.  poles  are  brought  together,  they  repel  each   other; 
hence  we  can  write  two  laws  governing  the  action  of  one  magnetic 
pole  on  another,  as  follows:  (1)  Like  magnetic  poles  repel  each 
other,  and  (2)  unlike  magnetic  poles  attract  each  other. 

The  action  of  the  compass  in  taking  a  north  and  south  position 
can  be  understood,  since  investigation  has  proved  that  the 
earth  is  a  gigantic  magnet,  having  one  magnetic  pole  near  the 
earth's  north  pole,  and  the  other  magnetic  pole  near  the  earth's 
south  pole. 

A  horseshoe  magnet  is  another  common  form  of  artificial 


26 


PRINCIPLES  OF  THE  TELEPHONE 


magnet,  one  .of  the  ends  being  a  N.  and  the  other  a  S.  pole,  as 
shown  in  Fig.  116. 

44.  Experiment  3. — Apparatus: 

Bar  Magnet. 

Nail. 

Piece  of  Watch  or  Clock  Spring  about  3  in.  long/  1 
(a)  Repeat  the  third  part  of  Experiment  1,  and  note  that  as 
long  as  the  magnet  and  nail  are  in  contact  the  point  of  the  nail 
will  hold  a  considerable  quantity  of  the  filings,  but  as  soon 
as  this  contact  is  broken  the  nail  loses  the  greater  part  of  its 
magnetism  and  most  of  the  filings  drop.  The  nail,  being  of  soft 
iron,  is  only  a  temporary  magnet. 


FIG.  12a. 


FIG.  126. 


(b)  Repeat  the  experiment  using  the  piece  of  watch  spring 
in  place  of  the  nails.  Even  after  the  contact  between  the 
spring  and  the  magnet  has  been  broken,  the  spring  retains 
the  greater  part  of  its  power  to  pick  up  the  filings.  The  spring, 
being  of  hard  steel,  has  become  a  permanent  magnet.  Try  the 
bar  magnet  used  in  this  experiment  with  a  file  to  see  whether 
or  not  it  is  of  hard  steel.  • 

45.  Permanent  and  Temporary  Magnets. — Artificial  magnets 
which  retain  their  magnetism  a  long  time  are  known  as  perma- 
nent magnets.  Wrought  iron  may  be  strongly  magnetized,  but 
as  soon  as  the  magnetizing  force  is  removed  it  loses  the  greater 
part  of  its  magnetism.  Hard  steel,  when  once  magnetized, 
will  retain  its  magnetic  properties  indefinitely.  Both  these  forms 
of  iron  are  made  use  of  in  telephone  work;  in  some  cases  we  use 
hard  steel  because  we  want  the  part  to  retain  its  magnetism,  as 
in  the  magnets  of  a  ringer,  and  in  other  cases  we  want  to  magnet- 
ize the  part  temporarily  and  then  want  the  same  part  to  lose  its 


MAGNETIC  PRINCIPLES  27 

magnetism  a  moment  later,  as  in  the  receiver  diaphragm.     The 
action  of  these  parts  will  be  explained  later. 

46.  Experiment  4. — Apparatus: 

Bar  Magnet. 

Piece  of  Watch  Spring. 

Be  sure  that  the  spring  used  in  Experiment  3  is  magnetized. 
After  testing  the  spring  by  dipping  in  the  filings,  cut  it  into  several 
short  pieces,  and  test  each  piece  for  magnetic  properties.  A 
magnetic  pole  will  exist  at  each  end  of  each  of  the  small  pieces, 
and  each  will  have  the  same  power  of  attraction  as  the  original 
magnet. 

47.  Magnetic  Lines. — The  property  by  which  a  magnet  will 


FIG.  13.  FIG.  14. 

attract  pieces  of  iron  or  other  magnetic  material  has  given 
rise  to  the  conception  of  magnetic  lines.  Magnetic  lines  are 
the  imaginary  lines  along  which  the  forces  of  attraction  and 
repulsion  are  exerted.  The  space  surrounding  a  magnet  in 
which  these  forces  are  exerted  is  known  as  the  magnetic  field. 
Each  individual  line  forms  a  complete  loop  or  circuit  passing 
through  the  poles  of  a  magnet,  and  the  number  of  these  lines 
which  pass  through  the  poles  determine  the  strength  of  the  mag- 
netic field. 

A  bar  magnet  is  surrounded  by  these  lines  which  enter  at 
one  pole  and  leave  through  the  other,  as  shown  in  Fig.  12a.  Mag- 
netic lines  are  considered  as  passing  out  of  the  magnet  at  the 
N.  and  into  the  magnet  at  the  S.  pole. 

The  distribution  of  magnetic  lines  under  different  conditions 
is  shown  in  Figs.  12a  and  126,  13,  and  14. 

A  magnet  can  not  be  produced  with  but  one  pole.  If  a  bar 
magnet  is  broken  into  a  number  of  small  pieces,  each  piece 
will  have  a  N.  and  a  S.  pole. 


28  PRINCIPLES  OF  THE  TELEPHONE 

48.  Experiment  5. — Apparatus: 

Horseshoe  Magnet. 

Bar  Magnet. 

Iron  Filings. 

Sheet  of  Smooth,  Stiff  Paper. 

(a)  Lay  the  bar  magnet  on  the  table,  and  over  it  place  a  sheet 
of  paper.  Sprinkle  iron  filings  over  the  paper.  Tap  the  paper 
gently  while  sprinkling  the  filings  and  note  that  the  arrangement 
of  the  filings  is  similar  to  the  diagram  of  the  magnetic  field  shown 
in  Fig.  12a. 

(6)  Repeat  the  above  experiment  using  the  horseshoe  magnet 
instead  of  the  bar  magnet. 

49.  The  Magnetic  Circuit. — The  path  of  the  magnetic  lines 
is  known  as  the  magnetic  circuit.  Thus  in  Fig.  12a  the  magnetic 
circuit  is  made  up  of  two  parts,  the  steel  of  the  magnet  and  the 
air  through  which  the  lines  pass.  A  magnetic  circuit  is  said 
to  be  closed  when  the  circuit  is  composed  entirely  of  magnetic 
substances,  such  as  iron  or  steel.  Whenever  pieces  of  iron  or 
steel  are  brought  into  a  magnetic  field,  the  lines  pass  through 
them  very  readily  and  they  become  magnetized.  If  the  material 
is  soft  iron,  when  taken  out  of  the  field  it  will  lose  most  of  its 
magnetism,  while  if  it  is  hard  steel  it  will  retain  its  magnetism, 
both  substances  behaving  the  same  as  if  they  had  been  in  actual 
contact  with  a  magnet. 

Magnetic  lines  tend  to  pass  along  the  path  offering  the  least 
resistance,  the  same  as  electric  currents.  Iron  and  steel  offer 
the  least  resistance  to  the  passage  of  magnetic  lines,  so  are  used 
for  magnetic  circuits  whenever  possible.  Air  offers  from  1 
to  10,000  times  as  much  resistance  to  magnetic  lines  as  iron, 
depending  upon  the  degree  of  magnetization.  Copper,  glass, 
paper,  and  other  nonmagnetic  substances  offer  the  same  resist- 
ance to  magnetic  lines  as  air.  Magnetic  circuits  through  sub- 
stances other  than  iron  are  usually  made  short,  so  that  the  number 
of  lines  in  the  magnetic  field  will  be  as  great  as  possible.  The 
horseshoe  magnet  is  stronger  than  the  bar  magnet  of  the  same 
size  because  the  magnetic  circuit  through  air  is  shorter.  That 
air  does  offer  considerable  resistance  to  magnetic  lines  can  be 
seen  from  the  fact  that  it  is  necessary  to  bring  a  magnet  quite 
near  a  piece  of  iron  before  any  attraction  is  noticed.  The  re- 
sistance which  any  substance  offers  to  the  passage  of  magnetic 
lines  is  known  as  reluctance. 


MAGNETIC  PRINCIPLES 


29 


50.  Electromagnetism. — Any  conductor  carrying  an  electric 
current  is  surrounded  by  a  magnetic  field,  as  shown  in  Fig.  15. 
The  dark  spot  in  the  center  of  the  figure  represents  a  cross- 
section  of  the  wire. 

51.  Experiment  6. — Apparatus: 

Two  Feet  of  Bare  Copper  Wire. 

Dry  Cells. 

Iron  Filings. 

Compass. 

(a)  Dip  the  wire  in  the  iron  filings  and  note  that  the  filings 
do  not  stick  to  the  wire.  Now  connect  the  ends  of  the  wire  to 
the  terminals  of  the  dry  cells;  place  the  wire  in  the  filings  and 


FIG.  15. 

observe  that  the  filings  stick  to  the  wire,  although  it  'is  not  a 
magnetic  substance.  Disconnect  one  end  of  the  wire  and  repeat 
the  experiment.  It  is  evident  that  .the  magnetic  field  exists  only 
as  long  as  the  current  is  flowing. 

(6)  Place  the  wire  over  the  compass  so  that  it  is  parallel  to 
the  needle  and  close  the  circuit.  Observe  that  the  needle  is 
deflected,  showing  the  magnetic  action  of  the  current.  Reverse 
the  current  through  the  wire  by  reversing  the  connections  to  the 
battery,  and  note  that  the  needle  is  deflected  in  the  opposite 
direction,  showing  the  magnetic  field  has  been  reversed. 

52.  Solenoids. — If  a  wire  carrying  a  current  be  wound  into 
a  coil,  as  shown  in  Fig.  16,  the  magnetic  lines  surrounding  each 
turn  of  the  coil  will  be  in  the  same  direction  as  those  of  the 
other  turns,  and  the  result  will  be  a  magnetic  field  similar  to 
that  of  a  cylindrical  bar  magnet.  A  coil  so  arranged  and  carry- 
ing a  current  is  called  a  solenoid.  A  solenoid  behaves  exactly 
like  a  bar  magnet.  At  one  end  of  the  solenoid  a  N.  pole  exists, 


30 


PRINCIPLES  OF  THE  TELEPHONE 


while  at  the  other  end  a  S.  pole  exists,  depending  upon  the  direc- 
tion of  the  current  flowing  in  the  wire.  A  reversal  of  the  current 
will  cause  a  reversal  of  the  magnetic  field. 

The  strength  of  the  magnetic  field  of  any  coil  depends  upon  the 
number  of  turns  in  the  coil  and  upon  the  current  flowing  in  the 
coil,  since  the  magnetic  field  of  a  solenoid  is  due  to  the  added 
effect  of  all  the  turns  in  the  coil. 
53.  Experiment  7. — Apparatus: 
Solenoid. 
Dry  Cells. 
Compass. 

(a)  Connect  the  solenoid  to  the  dry  cell  and  by  placing  one 
end  near  the  N.  pole  of  the  compass  observe  whether  it  attracts 
or  repels  the  compass. 


FIG.  16. 

If  it  attracts  the  N.  pole  of  the  compass,  what  kind  of  a  pole 
is  it?  If  it  repels?  Mark  the  end  with  an  N.  or  S. 

Now  test  the  other  end  the  same  way  and  observe  that  it  is 
of  opposite  polarity.  Mark  this  end  according  to  its  polarity. 

(6)  By  reversing  the  connections  at  the  battery,  cause  the 
current  to  flow  in  the  opposite  direction  through  the  coils. 

Test  as  before  with  the  compass  and  note  that  the  pole  which 
was  marked  N.  in  the  first  case  has  now  become  a  S.  pole,  and  the 
one  which  was  marked  S.  has  become  a  N.  pole. 

54.  Electromagnets. — If  an  iron  core  be  placed  in  a  solenoid, 
it  becomes  what  is  known  as  an  electromagnet.  Since  magnetic 
lines  pass  through  iron  much  more  readily  than  through  air, 
the  same  magnetizing  force  can  produce  a  stronger  field  through 
iron  than  through  air  or  some  other  nonmagnetic  substance. 


MAGNETIC  PRINCIPLES 


31 


Hence  the  purpose  of  the  iron  core  is  to  increase  the  strength  of 
the  magnetic  field  of  the  coil  without  increasing  the  current  or 
the  number  of  turns  in  the  coil. 


FIG.  17. 

The  electromagnets  used  in  telephone  work  are  of  three  general 
forms,  classified  according  to  the  form  of  the  iron  core. 

One  form,  the  bar  electromagnet,  consists  of  a  solenoid  wound 
on  a  straight  iron  core,  as  shown  in  Fig.  17.  An  examination 


FIG.  18. 

shows  that  the  magnetic  circuit  contains  a  long  air  gap.  If 
this  air  gap  be  shortened,  the  number  of  lines  and  therefore  the 
strength  of  the  magnetic  field  will  be  increased  without  changing 
the  current  or  coil  in  any  way. 


32 


PRINCIPLES  OF  THE  TELEPHONE 


55.  Horseshoe  Electromagnet. — One  of  the  easiest  ways  of  short- 
ening the  air  gap  is  to  bend  a  bar  electromagnet  in  the  form  of  a 
horseshoe,    as  shown  in  Fig.   18.     To    facilitate    manufacture, 
however,  the  core  of  the  horseshoe  electromagnet  is  usually  made 
in  three  parts  instead  of  being  bent  as  shown.     Fig.  19  shows  the 
general  form  of  commercial  horseshoe  magnets,  consisting  of  two 
spools  and  the  yoke  joining  their  cores.     Since  such  a  magnet 
usually  is  arranged  to  attract  an  armature,  the  latter  further 
decreases  the  air  gap,  as  shown,  to  the  short  spaces  between  the 
armature  and  poles. 

56.  The  Ironclad  Electromagnet. — The  ironclad  or  tubular 
electromagnet  is  shown  in  Fig.  20.     In  this  type,  the  coil  is  wound 


FIBER 


PAPER 


CORE. 


—  COIL 


ARMATURE 


FlG.    19. 


on  an  iron  core  and  is  surrounded  by  a  tubular  shell.  Such  a 
magnet  has  the  advantages  of  occupying  small  space,  and  of 
having  its  magnetic  field  confined  strictly  within  the  shell  so 
that  there  are  no  stray  lines  to  affect  other  apparatus  which  may 
be  near. 

57.  Construction  of  Electromagnets. — The  coils  for  electro- 
magnets are  usually  wound  in  the  form  of  spools.  Such  a  spool 
may  be  entirely  of  fiber  so  that  it  can  be  removed  from  the  core 
if  desirable,  or  the  fiber  ends  may  be  forced  on  the  iron  core  as 
shown  in  Fig.  18.  In  the  latter  form  several  layers  of  paper  are 
wrapped  around  the  core  to  insulate  it  thoroughly  from  the  coil. 
On  this  spool  the  insulated  wire  of  the  coil  is  wound  in  layers. 
Sometimes  the  layers  are  further  insulated  from  each  other  by 
a  thickness  of  paper. 


MAGNETIC  PRINCIPLES 


33 


CORE 


58.  Magnet  Wire. — The  insulated  copper  wire  used  in  wind- 
ing coils  for  electromagnets  is  known  as  magnet  wire. 

Most  magnet  wire  is  covered  with  either  silk  or  cotton.  Of 
the  two,  silk  has  the  higher  insulating  properties  and  is  used 
largely  on  very  fine  wire,  as  a  cover- 
ing of  silk  is  thinner  than  one  of 
cotton.  Cotton  is  used  almost  ex- 
clusively on  the  larger  sizes.  Silk 
or  cotton  insulated  wire  has  either 
one  or  two  layers  of  the  insulating 
materials,  and  is  known  as  single 
silk  (or  cotton)  covered,  and  double 
silk  (or  cotton)  covered.  When 
two  layers  are  used  they  are  wound 
in  opposite  directions.  As  both 
silk  and  cotton  absorb  moisture 
readily,  wire  insulated  with  these 
materials  is  sometimes  saturated 
with  melted  paraffine,  shellac,  var- 
nish, or  some  other  insulating  com- 
pound to  make  it  waterproof. 
More  often  the  coil  is  so  treated 
after  being  wound. 

Enameled  wire  is  a  later  develop- 
ment in  the  insulation  of  magnet 
wire.     Enameled  wire  is  made  by- 
coating  the  wire  with  liquid  enamel 
which  is  then  baked  on.     The  ad- 
vantages of  this  wire  are  that  it  is  FIG.  20. 
waterproof,    will   stand  high  tem- 
peratures, and  the  covering  of  insulating  material  is  very  thin. 

59.  Magnetic  Action  of  Receiver. — If  an  electric  current  be 
sent  through  the  coil  of  the  receiver  in  such  a  direction  that  the 
magnetic  lines  set  up  by  it  are  in  the  same  direction  as  those  of 
the  permanent  magnet,  the  strength  of  the  magnet  will  be  in- 
creased and  the  disk  will  be  drawn  closer  to  the  pole.     If  a  cur- 
rent be  sent  through  the  coil  in  the  opposite  direction,  however, 
so  that  the  magnetic  lines  due  to  the  current  oppose  those  of  the 
magnet,  the  strength  of  the  magnet  will  be  decreased  and  the 
diaphragm  will  spring  away  from  the  pole. 

When  a  fluctuating  current  flows  through  the  coil,  the  magnetic 


34  PRINCIPLES  OF  THE  TELEPHONE 

field  of  the  coil  will  increase  with  increasing  current  and  decrease 
as  the  current  decreases,  and  these  changes  will  cause  changes 
in  the  strength  of  the  field  of  the  permanent  magnet.  Thus, 
whether  or  not  the  lines  induced  by  the  coil  are  in  the  same 
direction  as  those  of  the  permanent  magnet,  there  will  be  changes 
in  the  strength  of  the  magnetic  field  whenever  there  are  varia- 
tions of  the  current  flowing  in  the  coil.  Hence  the  diaphragm 
will,  vibrate  in  harmony  with  the  changes  of  current. 

QUESTIONS 

1.  What  is  a  magnet?     What  is  magnetism? 

2.  What  is  an  artificial  magnet?     How  made? 

3.  What  is  the  pole  of  a  magnet?     How  many  poles  do  magnets  have? 

4.  What  is  a  compass  needle?     Which  is  the  N.  pole  of  a  magnet? 

5.  What  is  a  magnetic  substance?     Name  the  most  common  magnetic 
substance. 

6.  What  is  meant  by  magnetic  induction?     Give  an  illustration  of  mag- 
netic induction.     Can  magnetism  be  induced  in  a  bar  of  iron  without  having 
the  bar  come  into  contact  with  a  magnet? 

7.  Give  the  law  of  attraction  and  repulsion  between  magnet  poles. 

8.  What  is  a  permanent  magnet?     What  kind  of  material  is  usually  used 
in  permanent  magnets? 

9.  What  is  a  magnetic  line?     What  is  a  magnetic  circuit? 

10.  What  is  a  magnetic  field?     Upon  what  does  the  strength  of  a  magnet 
depend? 

11.  Which  offers  the  greater  resistance  to  magnetic  lines:    Air  or  steel? 
Steel  or  copper?     Wrought  iron  or  air? 

12.  Why  is  a  horseshoe  magnet  stronger  than  a  bar  magnet  of  the  same 
size? 

13.  If  a  telephone  receiver  is  examined  it  will  be  noticed  that  there  is  a 
steady  pull  between  the  disk  and  the  magnet.     How  can  this  be  explained? 

14.  What  would  be  the  effect  if  the  disk  and  magnet  poles  should  be 
moved  closer  together? 

15.  What  relation  exists  between  an  electric  current  and  magnetism? 

16.  What   is  a  solenoid?     Explain   how  a  solenoid   is  similar  to  a  bar 
magnet. 

17.  What  happens  when  the  current  stops  flowing  in  a  solenoid?     When 
the  current  is  reversed  what  happens?     Upon  what  does  the  strength  of  a 
solenoid  depend? 

18.  What  is  an  electromagnet?     Why  is  an  iron  core  used?     What  are 
the  common  forms  of  electromagnets?     Name  the  advantages  of  each. 

19.  How  is  magnet  wire  insulated?     Name  advantages  of  each  kind  of 
insulation. 

20.  Explain  how  a  fluctuating  electric  current  flowing  in  the  receiver  coil 
will  affect  the  diaphragm.     Explain  fully. 


CHAPTER  IV 
SOUND 

60.  Sound. — Sound  is  produced  by  the  vibration  of  some 
body,  and  is  transmitted  through  space  in  the  form  of  waves  in 
the  air;  hence  sound  may  be  defined  as  wave  motion  in  the  air, 
capable  of  affecting  the  sense  of  hearing. 

If  a  stone  be  dropped  into  a  pond  of  water,  a  disturbance 
is  set  up  which  spreads  in  the  form  of  waves  in  ever-widening 
circles. 

If  a  tuning  fork  be  started  vibrating,  sound  is  produced. 
The  sound  travels  from  the  source  of  disturbance  in  the  form 
of  air  waves.  Investigation  and  experiment  have  shown  that 
the  air  moves  forward  and  backward  in  the  direction  in  which 
the  sound  travels.  At  one  instant  the  air  in  front  of  the  fork 
is  condensed,  while  that  behind  it  is  rarefied,  and  the  next  in- 
stant the  air  in  front  of  the  fork  is  rarefied  while  that  behind  it 
is  compressed.  The  waves  thus  travel  as  a  series  of  compressions 
and  expansions.  The  sounds  which  issue  from  a  telephone 
receiver  are  caused  by  the  rapid  vibration  of  the  iron  diaphragm. 

Air  is  not  the  only  substance  that  will  transmit  sound  waves, 
water,  wood,  iron,  etc.,  being  useful  in  this  respect.  The  early 
telephone  experiments  mentioned  in  the  first  chapter  depended 
upon  the  transmission  of  sound  waves  through  a  tightly  stretched 
wire.  That  some  material  medium  is  necessary  for  the  trans- 
mission of  sound  waves  can  be  shown  by  placing  an  electric  bell 
under  the  receiver  of  an  air  pump  and  exhausting  the  air.  As 
the  air  is  exhausted  from  the  receiver,  the  sounds  from  the  bell 
grow  weaker  and  weaker  until  they  cease  entirely  when  the  air 
has  been  all  exhausted,  although  the  bell  may  be  seen  in  full 
operation  all  the  time. 

Since  vibrating  bodies  produce  sound  waves,  it  is  to  be  ex- 
pected that  sound  waves  are  capable  of  causing  certain  bodies  to 
vibrate  when  the  waves  come  into  contact  with  such  bodies. 
This  is  shown  by  the  fact  that  a  person  talking  in  a  room  where 
4  35 


36  PRINCIPLES  OF  THE  TELEPHONE 

there  is  a  piano  will  cause  certain  wires  of  the  instrument  to 
vibrate  and  thus  give  out  sounds.  Another  proof  is  that  a 
heavy  clap  of  thunder  will  often  cause  the  windows  of  a  house 
to  shake  violently.  In  the  telephone  the  sound  waves  of  the 
voice  are  directed  against  the  diaphragm  of  the  transmitter, 
causing  it  to  vibrate. 

61.  Velocity  of  Sound. — It  is  well  known  that  sound  waves 
take  a  considerable  amount  of  time  to  travel  from  one  point 
to   another.     Experiments  have  shown  that  sound  travels  at 
the  rate  of  about  1,090  ft.  per  second.     In  connection  with  this 
statement  it  is  interesting  to  compare  the  speeds  of  electricity 
and  light  with  that  of  sound.     Electric  waves  and  light  travel 
with  the'  same  speed,  which  is  in  round  numbers  186,000  miles 
a  second,  or  about  930,000  times  as  fast  as  sound,  since  sound 
travels  about  1  mile  in  5  sec.     That  light  travels  at  a  much 
higher  speed  than  sound  can  be  verified  easily  by  watching  a 
locomotive  at  a  distance  and  observing  how  long  a  time  is  re- 
quired for  the  sound  to  reach  the  ear  after  one  sees  the  steam 
issuing  from  the  whistle. 

62.  Properties  of  Sound. — The  properties  of  sound  depend 
upon   three   different   quantities:  pitch,    loudness,    and   timbre 
or  quality. 

63.  Pitch. — Pitch  is  determined  by  the  rate  of  vibration  of 
the  sounding  body;  that  is,  the  number  of  vibrations  per  second 
determine   whether   the   sounds   given   off   will   be   "high"    or 
"low,"  a  high  rate  of  vibration  giving  a  higher  pitched  sound 
than  a  low  rate  of  vibration.     The  short  wires  of  a  piano  give 
off  high-pitched  sounds  because  their  rate  of  vibration  is  rapid, 
and  the  longer  bass  strings  which  vibrate  at  a  slower  rate  give 
off  lower  tones. 

64.  Loudness. — Loudness  of  sound  depends  upon  the  distance 
through    which    the    sounding    body    vibrates.     The    distance 
through  which  the  vibrating  body  moves  is  called  the  ampli- 
tude.    Thus  when  a  piano  key  is  struck  a  sharp  blow,  the 
amplitude  of  the  string  will  be  greater  than  when  a  light  blow 
is  given  the  key.     In  the  former  case,  the  sound  is  louder  or 
stronger  than  in  the  second  case,  though  the  pitch  is  the  same. 
Loudness  depends  upon  the  energy  of  the  vibration. 

65.  Timbre  or  Quality. — Quality  is  that  property  of  sound 
not  due  to  pitch  or  loudness,  that  enables  us  to  tell  one  sound 
from  another.     For  an  example,  a  violin  and  piano  may  be  sound- 


SOUND  37 

ing  the  same  note,  yet  a  difference  in  quality  can  be  detected. 
This  difference  is  not  due  to  pitch  or  loudness.  The  char- 
acteristics of  the  waves  given  out  by  the  two  strings  are  dif- 
ferent. This  perhaps  can  be  made  clearer  by  considering  a 
water  wave.  When  such  a  wave  is  examined  it  is  seen  that 
many  small  waves  surmount  it.  Similarly  a  string  or  other 
sounding  body  can  start  waves  which  consist  of  a  fundamental 
wave  and  also  small  waves.  These  small  waves  are  called 
overtones,  and  so  change  the  wave  form,  and  thus  the  quality 
of  the  sound,  that  we  are  able  to  tell  one  person's  voice  from 
another's,  or  to  distinguish  between  the  sounds  of  different 
musical  instruments. 

66.  Transmission  of  Speech. — If  the  sounds  of  speech  were 
simply  in  the  form  of  waves  of  a  given  pitch,  they  could  be 
transmitted  over  the  telephone  lines  by  merely  opening  and 
closing   the    circuit    at   the   transmitter   the   required   number 
of  times  per  second.     For  every  time  the  circuit  was  opened 
or  closed  there  would  be  change  of  current  through  the  receiver 
and  a  corresponding  magnetic  action  which  would  cause  the 
diaphragm  to  move.     The  loudness  of  the  sound,  which  depends 
upon   the   amount   of   movement   of   the   receiver   diaphragm, 
could  be  controlled  by  variations  in  the  strength  of  the  current. 

However,  the  vibrations  due  to  the  sound  of  the  human 
voice  are  very  complex,  due  to  overtones  and  the  variations 
of  both  pitch  and  loudness  which  take  place  hundreds  of  times 
a  second.  Hence  to  transmit  such  sounds  is  much  more  difficult 
than  we  might  at  first  imagine,  since  the  current  flowing  in  the 
telephone  circuit  must  vary  with  the  slightest  variation  in  the 
sounds  to  be  transmitted,  whether  these  variations  be  in  timbre, 
pitch,  or  loudness. 

67.  Experiment  8. — Apparatus: 

Telephone  Receiver. 

Two  Dry  Cells. 

Copper  Wire. 

Coarse  File. 

(a)  Connect  one  dry  cell  in  series  with  the  receiver  as  shown 
in  Fig.  21.  Attach  one  of  the  wires  to  the  tang  of  the  file.  Draw 
the  end  of  the  other  wire  along  the  file  so  as  to  open  and  close  the 
circuit  repeatedly,  in  the  meantime  observing  that  the  sound 
given  off  by  the  receiver  is  merely  a  series  of  clicks,  which  occur 
whenever  the  circuit  is  opened  or  closed.  The  pitch  of  the  sound 


38 


PRINCIPLES  OF  THE  TELEPHONE 


depends  upon  the  rapidity  with  which  the  wire  is  drawn  along 
the  file. 

(6)  Repeat  the  above  with  two  cells  in  series,  and  note  that 
the  only  change  is  that  the  sound  produced  by  the  receiver  is 
louder  than  when  one  cell  is  used.  This  shows  that  the  loudness 
of  sounds  depends  upon  the  energy  of  the  vibrations  of  the 
diaphragm. 

68.  Variable  Resistance. — With  the  telephone  parts  connected 
as  shown  in  Fig.  22,  a  change  in  resistance  in  any  of  the  parts 
causes  a  change  in  the  current  flowing  in  the  circuit.  Hence, 
instead  of  opening  and  closing  the  circuit  to  send  variable  cur- 


FIG.  21. 

rents  over  the  line,  the  resistance  of  the  transmitter  is  changed 
from  time  to  time  by  the  sound  waves  of  the  voice.  This 
variable  resistance  is  obtained  by  the  use  of. carbon. 

Carbon  is  found  in  a  number  of  well-known  forms,  such  as 
charcoal,  graphite,  lampblack,  etc.  Hard  carbon,  similar  to 
arc-lamp  carbon,  is  used  in  telephone  transmitters.  The 
property  of  carbon  which  makes  it  suitable  for  this  work  is  that 
the  electrical  resistance  of  a  contact  made  of  this  substance  can 
be  regulated  by  the  pressure  applied.  This  resistance,  which 
depends  in  a  large  measure  upon  the  closeness  of  contact  of  the 
carbon  parts,  is  decreased  when  the  pressure  is  increased,  and 
increased  when  the  pressure  is  reduced.  Such  a  contact  is  very 
sensitive,  the  slightest  variation  in  pressure  causing  a  change  in 


SOUND 


39 


its  resistance.  In  the  transmitter  the  changes  in  pressure  neces- 
sary to  cause  variations  in  the  electrical  resistance  of  the  carbon 
parts  are  produced  by  the  vibrations  of  the  diaphragm,  and  since 
the  diaphragm  is  very  sensitive  and  responds  to  the  slightest 


FIG.  22. 


variations  of  pitch,  loudness,  and  quality  of  the  sound  waves 
of  the  voice,  the  pressure  on  the  carbon  parts  varies  according 
to  the  characteristics  of  these  sound  waves. 


CHAPTER  V 
TRANSMITTERS 

69.  The  Carbon  Transmitter. — In  the  earlier  forms  of  trans- 
mitters such  as  the  Edison  and  Blake,  the  variations  in  resist- 
ance were  obtained  through  the  action  of  the  diaphragm  on  a 
single  disk  of  carbon.     The  use  of  such  instruments  was  limited 
by  the  fact  that  currents  heavy  enough  to  give  the  required 
transmission  burned  the  surfaces  of  the  carbon  electrodes  at  the 
points  of  contact,  soon  destroying  them.     In  order  to  provide  a 
large  number  of  points  of  contact,  between  which  the  current  is 
divided,  the  granulated  carbon  type  of  transmitter  was  developed. 
This  type  is  used  at  present  to  the  exclusion  of  all  others,  and 
consists  of  two  carbon  disks,  one  stationary,  the  other  movable 
and  arranged  to  vibrate  with  the  diaphragm,  separated  by  a 
small  quantity  of  granulated  carbon.     The  greatest  drawback 
to  the  early  adoption  of  this  type  was  the  tendency  of  the 
granulated   carbon   to    "pack"   into   a   compact   mass,    which 
rendered  the  transmitter  useless.     In  order  to  overcome  this 
tendency  the  solid-back  type  was  developed. 

70.  White   Solid-back   Transmitter.— The   White   solid-back 
transmitter  which  has  been  the  standard  of  the  Bell  companies  for 
many  years,  is  shown  in  section  in  Fig.  23.     The  case,  A,  is  made 
of  brass,  having  a  heavy  cover,  B,  to  which  is  attached  the  hard- 
rubber  mouthpiece  M.     The  mouthpiece  serves  to  collect  the 
sound  waves  and  concentrate  them  upon  the  diaphragm,  D,  which 
is  a  thin  iron  or  aluminum  disk  having  its  edge  covered  with 
rubber,  R.     As  shown  in  Fig.  24,  two  springs,  S,  and  $',  bear  on 
the  diaphragm.     The  short  one  holds  the  edge  of  the  diaphragm 
firmly  against  the  cover,  B,  and  the  long  one  rests  on  the  dia- 
phragm to  dampen  its  vibrations  and  render  it  less  sensitive 
to  outside  noises. 

The  electrodes,  which  are  two  polished  carbon  disks,  E  and 
E',  are  contained  in  a  brass  chamber  consisting  of  two  parts. 
The  rear  electrode,  E,  which  is  the  larger  of  the  two,  is  firmly 
secured  within  the  brass  cup,  F.  The  cup,  F,  is  attached  to  the 

40 


TRANSMITTERS 


41 


FIG.  23. 


FIG.  24. 


42  PRINCIPLES  OF  THE  TELEPHONE 

bridge,  G,  by  means  of  the  pin  and  set-screw.  The  front  carbon 
is  fastened  to  the  stud,  0,  the  shank  of  which  passes  through  the 
diaphragm  and  is  held  in  place  by  two  check  nuts.  A  thin 
mica  washer,  M,  is  clamped  between  the  head  of  the  stud  and  the 
threaded  ring,  N,  the  outer  edge  of  this  washer  being  held  be- 
tween the  cap,  H,  and  the  cup,  F.  The  center  of  the  mica  washer 
is  therefore  rigidly  attached  to  the  front  electrode  and  partakes 
of  its  movements,  while  the  outer  edge  is  fastened  to  the  rear 
electrode  which  is  fixed.  Any  changes  in  relative  position  of 
the  electrodes  can  take  place  only  through  the  bending  of  the 
mica  washer.  In  addition  to  holding  the  front  electrode  in  its 
normal  position,  the  mica  washer  closes  the  chamber  containing 
the  electrodes  and  keeps  the  granulated  carbon  with  which  this 
space  is  filled  from  falling  out.  The  front  electrode  is  insulated 
from  the  frame  by  the  mica  washer,  and  by  the  fiber  lining,  L, 
which  keeps  the  granulated  carbon  away  from  the  sides  of  the 
cup.  Since  one  terminal  is  connected  to  the  front  electrode  by 
the  flexible  connection,  C  (see  Fig.  24)  and  the  other  to  the  frame 
of  the  transmitter,  any  current  which  passes  through  the  instru- 
ment must  flow  through  the  granulated  carbon. 

The  operation  of  the  solid-back  transmitter  is  as  follows: 
The  sound  waves  of  the  voice  of  the  person  speaking  cause 
vibration  of  the  diaphragm,  which,  being  rigidly  connected  to 
the  front  electrode,  causes  that  to  vibrate  also,  as  the  mica  washer 
which  holds  it  in  place  is  very  flexible.  Since  the  back  electrode 
is  held  stationary,  the  granulated  carbon  is  subjected  to  varia- 
tions in  pressure.  As  a  result  the  current  flowing  through  the 
transmitter  is  varied. 

71.  New  Western  Electric  Transmitter.— The  new  Western 
Electric  transmitter,  shown  in  Fig.  25,  is  a  modified  form  of  the 
White  instrument.  As  in  the  White  transmitter, .  the  front 
electrode  is  carried  on  a  mica  washer  and  is  connected  by  a 
stud  to  the  center  of  the  diaphragm,  and  the  rear  electrode  is 
fixed  in  the  bottom  of  the  electrode  chamber.  This  chamber 
is  attached  to  the  back  of  a  metal  cup,  S  (which  takes  the  place 
of  the  bridge  in  the  White  transmitter)  by  the  threaded  part,  C. 
This  not  only  holds  the  chamber  in  place,  but  also  holds  the 
outer  edge  of  the  mica  washer  firmly  between  the  two  parts. 

The  metal  cup  and  diaphragm  are  insulated  from  the  shell 
of  the  transmitter  at  R,  so  that  neither  of  the  electrodes  is 


TRANSMITTERS 


43 


FIG.  25. 


FIG.  26. 


44 


PRINCIPLES  OF  THE  TELEPHONE 


connected  to  the  exposed  metal  parts.  Of  the  terminals,  shown 
in  the  figure,  TI  is  connected  to  the  cup,  and  the  other,  Tz, 
which  is  insulated  from  the  cup,  is  connected  to  the  front  electrode 
by  a  flexible  connection. 

72.  Kellogg  Transmitter. — The  Kellogg  Switchboard  and 
Supply  Co.'s  transmitter  is  shown  in  section  in  Fig.  26.  It  will 
be  immediately  noticed  that  the  chief  difference  between  this 
instrument  and  those  previously  discussed  is  that  the  electrode 
cup  is  made  a  part  of  the  diaphragm,  D,  and  therefore  partakes 


FIG.  27. 

of  its  movements.  In  order  that  the  moving  parts  may  not  be 
too  heavy  to  respond  readily  to  the  sound  waves  of  the  voice, 
the  diaphragm  is  made  of  hard-drawn  aluminum  with  the  elec- 
trode chamber  stamped  in  its  center.  The  diaphragm,  D,  the 
front  carbon  disk,  E,  which  is  attached  to  the  bottom  of  the 
chamber,  the  granulated  carbon,  C,  and  the  mica  washer,  M, 
are  the  movable  parts.  The  disk  E'  is  stationary,  as  it  is  rigidly 
attached  to  the  bridge,  G.  This  bridge  is  a  straight  piece  of 
hard-drawn  brass. 

To  prevent  the  transmitter's  taking  up  outside  noises'  and 
being   affected   by   mechanical    vibration   which   might   inter- 


TRANSMITTERS 


45 


fere  with  talking,  the  diaphragm  rests  on  a  soft  pad,  P.  Two 
damping  springs  having  cushioned  tips  have  been  provided  as 
in  the  White  instrument.  The  working  parts  of  this  trans- 
mitter are  all  insulated  from  the  case. 

73.  Monarch  Transmitter. — The  Monarch  transmitter  shown 
in  Fig.  27  differs  from  those  already  studied  in  having  both 
its  electrodes  mounted  on  flexible  mica  washers  which  support 
the   carbon  chamber.     The  rear  electrode,   which  is  attached 
to  the  bridge,  is  the  only  fixed  part.     The  diaphragm  is  of 
aluminum  and  is  separated  from  the  case  by  an  insulating  ring. 
The  flexible  connection  between  one  terminal  and  the  front 
electrode  is  shown  in  the  figure.     The     ^_____________, 

stud  of  the  rear  electrode,  which  is 

insulated  from  the  bridge,  is  connected 
to  the  other  terminal. 

74.  Operator's  Transmitter. — In 
Fig.  28   is  shown  a  special   form  of 
transmitter  for  switchboard  operators' 
use.     As  this  instrument  is  provided 
with  a  plate  which  rests  on  the  oper- 
ator's breast,  the  long  curved  mouth- 
piece is  always  in  the  proper  position 
for  use.     The  breast  transmitter  and 
watch-case  receiver  described  in  the 
next  chapter  make  up  the  operator's  set. 

In  cases  where  the  operator  is  compelled  to  leave  the  switch- 
board frequently  to  attend  to  other  duties,  as  in  small  exchanges, 
many  of  the  advantages  of  the  breast  transmitter  are  lost. 
In  such  cases  a  transmitter  of  the  same  form  as  the  subscribers' 
instrument  is  suspended  by  adjustable  cords  in  front  of  the 
operator. 

75.  Carbon  Electrodes.— The  disks  for  use  in  transmitters 
are   made   of   specially   prepared   hard   carbon.     The  faces  in 
contact  with  the  granulated  carbon  are  made  as  nearly  true 
as  possible,   and   are  highly  polished.     The  reverse  sides  are 
copper  plated  and  then  soldered  to  the  backing  plates  of  brass. 
When  assembled,  the  electrodes  must  be  parallel  to  each  other 
if  good  results  in  operation  are  to  be  obtained. 

The  granular  carbon  is  very  hard,  uniform  in  size,  and  free 
from  dust.  As  mentioned  above,  a  great  deal  of  trouble  was 
caused  by  the  packing  of  the  granulated  carbon  in  the  earlier 


FIG.  28. 


46  PRINCIPLES  OF  THE  TELEPHONE 

transmitters,  due  to  moisture,  unevenness  in  size  of  carbon 
grains,  and  by  wedging  apart  of  the  carbon  disks.  These 
difficulties  were  overcome  by  making  the  chamber  containing 
the  carbon  grains  waterproof;  by  making  the  grains  of  uniform 
size  and  hard  enough  not  to  crush  in  service;  and  by  improve- 
ments in  manufacture,  so  that  the  electrodes  are  always  parallel 
to  each  other. 

Any  transmitter  can  be  packed  by  pulling  the  diaphragm 
forward  so  as  to  widely  separate  the  electrodes.  This  allows 
the  carbon  granules  to  settle  and  wedge  the  electrodes  apart. 
In  the  earlier  types  this  could  be  done  by  placing  the  lips  against 
the  mouthpiece  and  drawing  in  the  breath.  In  order  to  prevent 
this,  modern  mouthpieces  are  slotted  at  the  base. 

According  to  a  recent  report  of  the  American  Telephone 
and  Telegraph  Co.  there  were  designed,  constructed,  and  in- 
stalled, during  the  37  years  from  1877  to  1914,  53  improved  types 
and  styles  of  telephone  receivers  and  73  types  and  styles  of 
transmitters.  These  figures  do  not  include  hundreds  of  minor 
improvements  made  in  both  transmitters  and  receivers. 


QUESTIONS 

1.  How  are  sounds  transmitted  by  the  telephone?     Does  sound  actually 
travel  from  one  instrument  to  the  other? 

2.  What  are  the  parts  of  the  telephone  used  in  transmitting  the  sounds  of 
speech? 

3.  Will  a  telephone  work  if  the  battery  be  removed?     Why  not? 

4.  What  do  you  understand  sound  to  be?     How  is  sound  produced? 
How  transmitted  from  place  to  place? 

5.  How  fast  does  sound  travel  through  air?     Compare  the  speed  of  sound 
with  that  of  light.     With  electricity. 

6.  Upon  what  three  things  does  the  quality  of  sound  depend? 

7.  What  is  pitch?     Loudness?     Timbre? 

8.  Explain  how  a  telephone  receiver  produces  sound. 

9.  Explain  how  sound  waves  can  cause  the  transmitter  diaphragm  to  move. 

10.  What  are  the  characteristics  of  the  waves  set  up  by  the  sounds  of 
speech? 

11.  Why  can  not  the  sounds  of  speech  be  transmitted  by  repeatedly  open- 
ing and  closing  the  telephone  circuit  as  in  the  experiment? 

12.  Examine  carefully  as  many  different  makes  of  transmitters  as  pos- 
sible.    What  differences  do  you  find? 

13.  Explain  how  changes  in  transmitter  resistance  can  cause  the  receiver 
to  operate.     How  are  these  changes  in  resistance  caused? 

14.  Why  is  carbon  used  in  transmitters? 


TRANSMITTERS  47 

15.  Explain  briefly  the  construction  and  action  of  the  solid-back  trans- 
mitter. 

16.  What  is  meant  by  packing  of  a  transmitter?     How  is  packing  caused ? 

17.  Does  the  carbon  transmitter  ever  open  the  battery  circuit?     Answer 
this  question  by  studying  the  transmitters  shown  in  Figs.  23  to  27. 


CHAPTER  VI 
RECEIVERS  AND  INDUCTION  COILS 

76.  The  Receiver. — The  telephone  receiver  makes  use  of  the 
fluctuating  electric  currents  to  reproduce  the  sound  waves  which 
caused  these  current  variations  at  the  transmitting  end  of  the 
line.     Receivers  are  electromagnetic  in  their  action,  as  has  been 
briefly  explained  in  an  earlier  chapter. 

77.  Early  Receivers. — Early  receivers  were  of  the  single-pole 
type;  that  is,  the  diaphragm  was  influenced  by  only  one  pole  of 
the  magnet.     An  early  form  of  receiver  is  shown  in  Fig.  10,  the 
parts  being  named.     The  operation  of  such  a  receiver  is  due  to 
the  magnetic  action  of  the  current  flowing  through  the  coil, 
which  either  weakens  or  strengthens  the  magnetic  field  of  the 
permanent  magnet,  and  thus  causes  the  diaphragm  to  vibrate 
in  unison  with  the  changes  of  current  strength. 

The  magnetic  circuit  of  this  type  of  receiver  contains  a  very 
long  air  path ;  hence  a  considerable  current  is  required  to  produce 
the  required  changes  in  magnetic  force.  Another  serious  objec- 
tion to  this  type  of  receiver  is  the  ease  with  which  the  adjustment 
is  disturbed,  owing  to  the  magnet  being  attached  to  the  shell 
at  the  end  farthest  from  the  diaphragm. 

78.  Induced  Electric  Pressure. — A  further  investigation  of  the 
relations  existing  between  magnetism  and  electricity  shows  that 
when  a  wire  is  moved  in  a  magnetic  field  so  as  to  cut  the  magnetic 
lines,  an  electrical  pressure  is  set  up  in  the  wire.     The  value  of 
this  pressure  depends  upon  the  rate  of  cutting  the  magnetic 
lines,  or,  in  other  words,  the  number  of  lines  cut  per  second. 

A  pressure  generated  by  the  relative  movement  of  a  conductor 
and  a  magnetic  field,  is  called  an  induced  pressure. 

The  direction  of  the  induced  pressure  depends  upon  the  direc- 
tion of  the  cutting  of  magnetic  lines.  Hence  a  movement  of  a 
conductor  in  one  direction  through  a  magnetic  field  will  cause 
a  pressure  in  one  direction,  and  a  movement  in  the  opposite 
direction  will  generate  a  pressure  in  the  opposite  direction.  A 
pressure  can  be  induced  in  a  coil  by  changing  the  strength  of  the 


RECEIVERS  AND  INDUCTION  COILS  49 

magnetic  field  inside  the  coil.  Since  a  magnetic  line  makes  a 
complete  loop  or  path,  it  is  evident  that  if  the  number  of  lines 
inside  a  coil  are  changed,  some  lines  must  be  cut  by  the  coil 
during  the  change.  Increasing  the  strength  of  the  field  inside 
a  coil  sets  up  a  pressure  in  one  direction,  while  decreasing  the 
number  of  lines  sets  up  an  opposite  pressure,  because  the  lines 
are  cut  in  opposite  directions  during  these  changes. 

The  induced  pressure  will  be  maintained  only  so  long  as  the 
relative  motion  of  conductor  and  field  is  kept  up,  or  while 
magnetic  lines  are  being  cut. 

In  general  we  may  say  that  whenever  the  magnetic  field  sur- 
rounding a  conductor  varies  in  intensity  an  electrical  pressure 
will  be  set  up  in  the  conductor,  and  if  the  circuit  be  closed  a 
current  will  flow. 

Induced  pressure  may  be  either  direct  or  alternating,  depend- 
ing upon  whether  the  magnetic  lines  are  cut  continuously  in  one 
direction  or  the  direction  of  cutting  is  reversed  from  time  to 
time. 

79.  Direct  Current. — A  direct  current  flows  continuously  in 
one  direction,  although  its  strength  may  vary  from  time  to  time. 
The  flow  of  a  current  of  electricity  caused  by  the  pressure  of  a 
battery  is  in  one  direction. 

Direct  currents  may  be  divided  into  two  classes,  continuous 
and  pulsating.  A  continuous  current  is  one  the  strength  of 
which  does  not  change  materially  from  instant  to  instant.  A 
pulsating  current,  however,  is  a  direct  current  the  strength  of 
which  may  vary  from  time  to  time  without  change  in  the  direc- 
tion of  flow.  Continuous  currents  are  used  for  lighting  and  power 
purposes.  Pulsating  currents  are  made  use  of  in  telephone 
practice. 

80.  Alternating    Currents. — An    alternating    current    is    one 
which  varies  continuously  in  strength  and   changes  direction 
periodically. 

81.  Experiment  9. — Apparatus: 

Two  Telephone  Receivers. 
About  50  ft.  of  Annunciator  Wire. 

Connect  two  telephone  receivers  by  about  25  ft.  of  copper  wire. 
Have  a  person  in  another  room  to  assist  you  and  see  if  sounds 
can  be  transmitted  without  using  any  batteries  in  the  circuit. 

It  will  be  seen  from  the  above,  since  no  battery  or  other  source 
of  power  is  used,  that  the  only  energy  used  in  operating  this 


50  PRINCIPLES  OF  THE  TELEPHONE 

telephone  is  that  of  the  sound  waves  themselves.  This  energy 
is  very  small;  hence  the  resultant  current  sent  from  one  station 
to  another  is  likewise  small,  and  sounds  can  be  transmitted 
only  a  short  distance.  It  was  early  realized  by  those  interested 
in  the  development  of  the  telephone  that  if  the  telephone  was  to 
become  of  any  commercial  value,  one  capable  of  transmitting 
speech  to  a  greater  distance  was  necessary. 

82.  The  Receiver  as  a  Transmitter. — Two  receivers  connected 
as  shown  in  Fig.  29  formed  the  first  practical  telephone  for  the 
transmission  of  speech,  and  constituted  Bell's  invention.  The 
operation  of  such  a  telephone  is  as  follows: 

Suppose  that  A  is  the  sending  or  transmitting  station,  and  B 
the  receiving  station.  The  sound  waves  due  to  the  sounds  of 
speech  strike  the  diaphragm  at  A  and  cause  it  to  vibrate  in  unison 


FIG.  29. 

with  the  waves  of  sound.  That  is,  every  variation  in  the  pitch, 
loudness,  or  timbre  of  the  sounds  affects  the  diaphragm.  The 
vibrations  of  the  diaphragm  cause  variations  in  the  strength  of 
the  magnetic  field,  since  every  vibration  causes  a  change  in  the 
length  of  the  air  gap  between  the  disk  and  the  pole  of  the  magnet, 
and  thus  increases  or  decreases  the  number  of  magnetic  lines 
which  pass  through  the  coil. 

Every  time  the  magnetic  field  is  disturbed,  induced  currents 
are  set  up  in  the-  coil.  These  electrical  currents  flowing  through 
the  coil  at  B  cause  the  diaphragm  at  B  to  vibrate  in  unison  with 
that  at  A,  and  thus  produce  sound  waves  like  those  which 
cause  the  diaphragm  at  A  to  vibrate. 

The  receiver  seemed  to  be  quite  satisfactory,  for  it  would  work 
when  large  enough  currents  could  be  made  to  flow  through  it. 
Hence  efforts  were  made  to  improve  the  transmitter,  which  re- 
sulted in  the  development  of  the  carbon  transmitter.  We  have 
observed  that  the  carbon  transmitter  does  not  generate  its  own 
current,  but  merely  controls  the  current  from  some  outside 
source,  such  as  batteries. 

83.  Bipolar  Receiver. — In  the  bipolar  type  of  receiver  the  air 
gap  is  very  much  shorter  than  in  the  single-pole  receiver,  since 


RECEIVERS  AND  INDUCTION  COILS  51 

both  poles  are  near  the  diaphragm.  The  working  parts  of  the 
receiver  are  attached  to  the  case  near  the  diaphragm,  or  are 
arranged  in  an  inner  metallic  case  so  that  the  adjustment  is 
independent  of  the  outer  case. 

84.  Western  Electric  Receiver. — Fig.  30  shows  the  con- 
struction of  the  Western  Electric  bipolar  receiver.  The  shell  is 
of  hard  rubber  and  is  made  in  three  parts.  Two  permanent  bar 
magnets  are  employed,  being  fastened  together  so  as  to  form  a 
single  horseshoe  magnet.  Two  soft-iron  pole  pieces  P  and  P' 
are  attached  to  the  ends  of  the  magnet  near  the  diaphragm. 
Each  one  of  the  soft-iron  poles  is  surrounded  by  a  coil  of  very 
fine  insulated  copper  wire,  marked  M  and  M'  in  the  figure.  Im- 
mediately in  front  of  the  poles  is  placed  the  sheet-iron  dia- 
phragm D  which  must  not  touch  the  pole  pieces  even  when 
vibrating  through  its  widest  range.  One  of  the  magnet  poles  is 
N.  and  the  other  is  S.  The  diaphragm  forms  a  part  of  the 
magnetic  circuit,  and  where  the  lines  enter  the  diaphragm  a 
S.  pole  is  formed,  and  where  the  lines  leave  the  diaphragm  a 
N.  pole  is  formed.  Thus  the  diaphragm  acts  as  an  armature 
and  by  the  attraction  of  the  magnet  is  constantly  bent  or  dished 
toward  the  pole  pieces. 

The  coils  on  the  pole  pieces  are  connected  so  that  the  mag- 
netic lines  set  up  by  a  current  passing  through  them  will  make 
one  a  N.  pole  and  the  other  a  S.  pole.  The  currents  flowing 
through  the  coils  in  one  direction  tend  to  strengthen  the  field  of 
the  permanent  magnet,  and  currents  flowing  in  the  opposite 
direction  tend  to  weaken  the  field  of  the  permanent  magnet. 
The  diaphragm  will  spring  away  from  the  pole  pieces  when  they 
are  weakened,  and  when  the  current  ceases  the  diaphragm  will 
be  drawn  back  toward  the  pole  pieces.  When  the  magnetic 
field  set  up  by  the  coils  assists  the  field  of  the  magnet,  the 
diaphragm  will  be  drawn  nearer  to  the  pole  pieces,  and  when  the 
current  stops  the  diaphragm  will  again  spring  back  to  its  normal 
position. 

From  this  it  will  be  seen  that  if  a  current  flows  first  in  one 
direction  and  then  in  another,  or  an  alternating  current  flows  in 
the  receiver  coil,  the  diaphragm  will  answer  every  impulse  of 
current,  no  matter  from  which  direction  it  comes.  Alternating 
currents  flow  in  circuits  where  an  induction  coil  is  used. 

If  the  receiver  were  not  equipped  with  a  permanent  magnet, 
its  magnetic  field  would  be  strengthened  by  a  current  flowing 


52 


PRINCIPLES  OF  THE  TELEPHONE 


through  the  coil  in  either  direction,  and  the  diaphragm  would  be 
attracted  or  drawn  in  toward  the  pole  whenever  a  current 
flowed.  However,  an  alternating  current  flowing  in  the  receiver 


FIG.  30. 


FIG.  30a. 


coil  will  alternately  strengthen  and  weaken  the  field  of  a  perma- 
nent magnet,  in  the  first  case  drawing  the  diaphragm  out  of  its 


FIG.  306. 


normal  position,  closer  to  the  pole,  and  when  the  field  is  weak- 
ened, allowing  it  to  spring  farther  away.  Hence,  when  a  perma- 
nent magnet  is  used,  an  alternating  current  is  capable  of  pro- 


RECEIVERS  AND  INDUCTION  COILS  53 

ducing  a  greater  vibration  of  the  diaphragm  than  would  be  the 
case  if  a  soft-iron  core  were  used.  The  pitch  is  also  an  octave 
lower. 

The  resistance  of  the  coils  M  and  M '  is  usually  about  60  ohms 
for  the  pair,  or  30  ohms  for  each  coil. 

The  magnet  is  attached  to  the  case  by  means  of  a  threaded 
block  which  screws  into  the  internal  thread  B.  This  arrangement 
allows  of  close  adjustment  of  the  distance  between  the  pole 
faces  and  the  disk,  and  as  a  result  of  the  close  coupling,  changes 
in  temperature  do  not  readily  affect  this  adjustment.  -  The 
latest  type  of  this  receiver  no  longer  has  exposed  binding  posts. 
This  is  shown  in  Fig.  30a  and  b.  This  receiver  is  the  standard 
in  use  by  the  Bell  Co. 

85.  The  Kellogg  Receiver. — The  Kellogg  receiver  with  in- 
ternal binding  posts  for  the  wires,  or  cords,  as  they  are  com- 
monly called,  is  shown  in  section  in  Fig.  31.  The  shell,  S,  and 


FIG.  32. 

the  cap,  E,  are  of  composition  rubber,  the  shell  consisting  of  a 
single  piece.  In  order  to  make  the  cap  stronger  and  less  liable 
to  split  under  hard  usage,  a  perforated  copper  disk  is  molded  into 
it.  The  diaphragm,  D,  is  firmly  clamped  between  the  cap  and 
the  brass  cup,  C,  to  which  the  permanent  magnets  are  at- 
tached. Therefore,  the  adjustment  which  is  made  between  the 
diaphragm  and  the  poles  of  the  magnet  at  the  time  of  manu- 
facture is  permanent. 

The  receiver  can  be  completely  taken  apart,  without  breaking 
any  connections,  by  removing  the  cap.  Fig.  32  shows  the  re- 
ceiver with  shell  removed.  The  permanent  magnets,  P  and 
Pf,  Fig.  31,  are  placed  side  by  side  with  corresponding  poles  at 
opposite  ends  and  bolted  together  at  the  rear  end,  holding  the 
block  of  iron,  H,  firmly  between  them,  in  effect  forming  a  U 


54 


PRINCIPLES  OF  THE  TELEPHONE 


I 


magnet.  At  the  diaphragm  end  the  soft-iron  pole  pieces  are 
attached.  The  two  pole  pieces  are  separated  by  a  part  of  the 
brass  cup,  and  are  firmly  clamped  between  the  permanent  mag- 
nets by  the  brass  bolt,  B.  Brass  being  a 
nonmagnetic  substance,  as  has  been 
shown  above,  has  no  effect  on  the  mag- 
netic field  of  the  magnet.  In  order  that 
no  strain  may  be  placed  upon  the  bind- 
ing posts  when  the  receiver  is  in  use,  the 
cord  is  firmly  tied  to  the  block,  H. 

As  receiver   cords  are  a  considerable 
source  of  annoyan.ce,  it  is  interesting  in 
connection  with  this  receiver  to  note  the 
F      33  method  of  fastening  the  metal  tips  to 

the  flexible  strands  of  the  cords.     Fig. 

33  shows  the  details  of  making  this  connection.  The  cord  tip 
is  first  wrapped  tightly  with  wire,  .and  the  strands  are  brought 
back  over  this  and  firmly  held  in  place  by  a  metal  clamp,  as 


FIG.  34. 


shown.     The  tip,  which  is  turned  and  bored  from  a  solid  brass 
rod,  is  then  soldered  over  this  special  clamp. 

The  particular  advantages  of  a  receiver  with  internal  binding 
posts  are  that  the  binding  posts,  being  inside  the  case,  are  not 


RECEIVERS  AND  INDUCTION  COILS  55 

subject  to  injury;  the  cord  at  point  of  contact  is  not  subject  to 
damage;  the  user  of  the  receiver  cannot  receive  shocks  from  the 
same,  since  he  can  not  touch  the  posts;  and  the  receiver  has  a 
very  neat  appearance. 

86.  Operator's  Receiver. — The  operator's  receiver  (or  watch- 
case  receiver,  as  it  is  often  called  on  account  of  its  shape)  is 
shown  in  Figs.  34  and  35. 

This  instrument  is  a  double- 
pole  receiver;  hence  the  opera- 
tion is  the  same  as  that  of  the 
hand  receivers  described  above. 
The  permanent  magnet  consists 
of  steel  rings,  P,  which  are  cross- 
magnetized  so  that  a  N.  pole  ex- 
ists  on  one  side  and  a  S.  pole  on 

the  other.  The  soft-iron  pole  pieces  are  clamped  between  the 
bottom  ring  and  the  case,  as  shown  in  the  sectioned  view. 

87.  Sensitiveness  of  Receivers. — The  sensitiveness  of  a  re- 
ceiver depends  upon  the  strength  of  the  permanent  magnet  and 
upon  the  diameter  and  thickness  of  the  diaphragm,   and  its 
distance  from  the  magnet  poles.     A  thin  diaphragm  responds 
very  readily  to  currents  of  high  frequency  (or  rapid  vibration) 
and  gives  clear  and  sharp  tones,  while  a  thicker  one  is  more 
rigid  and  responds  readily  only  to  those  currents  having  low 
frequency.     About  )-{  oo  in-  is  the  average  thickness  of  sheet  iron 
used  in  receiver  diaphragms,  the  diameter  being  about  2J£  in. 
For  successful  operation  a  thick  diaphragm  must  be  larger  than 
a  thin  one.     The  chief  objection  to  a  very  sensitive  receiver  is 
that  it  reproduces  any  disturbances  which  the  line  may  have 
taken  up  as  faithfully  as  it  does  the  sound  from  the  transmitting 
end.     A  very  sensitive  receiver,  therefore,  would  be  unsuitable 
for  use  on  a  grounded  line  or  on  a  metallic  circuit  of  poor  con- 
struction.    On  a  long  metallic  circuit  of  good  construction,  the 
simple  weakening  of  the  transmitter  current  may  be  compensated 
to  some  extent  by  using  a  sensitive  and  delicate  receiver. 

To  secure  loudness,  the  receiver  must  be  arranged  so  that  the 
alternating  currents  from  the  line  produce  the  largest  possible 
movement  of  the  diaphragm  in  both  directions  from  its  normal 
position.  The  strength  of  the  permanent  magnet  must  be  de- 
signed with  reference  to  the  properties  of  the  diaphragm  and  <the 
strength  of  the  line  currents.  If  the  permanent  magnet  be  too 


56  PRINCIPLES  OF  THE  TELEPHONE 

strong,  the  diaphragm  will  be  dished  excessively,  and  no  con- 
siderable movement  is  possible  when  the  magnet  is  made  still 
stronger,  although  a  large  movement  takes  place  in  the  opposite 
direction  when  the  magnet  is  weakened.  If  the  permanent 
magnet  be  too  weak,  the  movement  of  the  diaphragm  will  be 
large  when  the  magnet  is  strengthened,  and  small  when  it  is 
weakened.  Hence,  if  the  permanent  magnet  be  either  too  strong 
or  too  weak,  the  receiving  will  be  imperfect. 

88.  Direct-current  Receiver. — A  later  development  in  receiver 
construction  is  the  direct-current  receiver. 

In  common  battery  telephone  practice  when  the  line  is  in  use 
there  is  a  steady  current  flowing  from  the  central  office  to  the 
subscriber's  instrument  to  energize  the  transmitter  of  the 
latter.  In  the  direct-current  receiver  this  current  is  made  to  flow 
through  the  windings  of  the  receiver,  thus  producing  a  magnetic 
field  which  takes  the  place  of  that  due  to  the  permanent  magnets 
in  the  common  receiver.  Hence  no  permanent  magnets  are  re- 
quired. This  action  will  be  more  fully  understood  when  the 
common  battery  system  is  studied  in  a  later  chapter. 

89.  The  Automatic  Electric  Co.'s  Direct-current  Receiver.— 
The  Automatic  Electric  Co.'s  direct-current  receiver  is  shown  in 


FIG.  36. 

Fig.  36.  The  working  parts  of  the  receiver  are  contained  within 
the  brass  cup,  S.  The  winding  consists  of  a  single  coil  mounted 
on  the  core,  C,  which  in  turn  is  attached  to  the  center  of  a  U- 
shaped  iron  stamping  having  its  ends,  P  and  P',  bent  up  and 
partially  surrounding  the  coil. 

The  direct  current  flowing  in  the  line  magnetizes  the  core  and 
steel  stamping.  From  the  shape  of  the  magnetic  circuit,  it  is 
evident  that  at  any  given  time  the  end  of  the  core  near  the  dia- 


RECEIVERS  AND  INDUCTION  COILS 


57 


phragm  will  be  a  N.  pole,  and  the  ends,  P  and  Pf,  will-be  S.  poles, 
or  vice  versa.  This  receiver,  then,  has  all  the  advantages  of  the 
bipolar  receiver  in  having  both  its  poles  near  the  diaphragm. 
When  the  talking  circuit  is  open  there  is  no  current  flowing  in  the 
coil,  and  the  diaphragm  will  be  perfectly  flat  at  a  short  distance 
from  the  poles.  However,  as  soon  as  the  talking  circuit  is  closed, 
direct  current  flows  through  the  coil  and  draws  the  diaphragm 
toward  the  poles  of  the  magnet,  exactly  as  is  done  in  the  case  of 
the  polarized  receivers.  When  the  fluctuating  voice  currents 
flow  through  the  coil  they  will  either  strengthen 
or  weaken  the  direct  current  flowing  in  the 
coil,  and  will  cause  the  magnetic  field  to  fluc- 
tuate, thus  causing  a  movement  of  the  dia- 
phragm. This  receiver  is  designed  to  operate 
when  current  values  range  from  0.45  to  0.80 
amp. 

90.  The  Monarch  Direct-current  Receiver. 
— The  Monarch  direct-current  receiver  shown 
in  Fig.  37  operates  on  quite  a  different  plan 
than  the  one  just  discussed.     This  receiver 
has,  instead  of  permanent  magnets,  two  soft- 
iron  cores,  S  and  /S',  on  which  are  mounted 
two  long  coils,  C  and  C'.     Soft-iron  pole  pieces 
are  attached  to  the  ends  of  these  cores,  as 
shown,  and  carry  the  two  coils,  M  and  M1 '. 

The  two  sets  of  coils  are  connected  in  parallel  to  the  cords  of  the 
receiver.  The  coils  C  and  C'  have  a  somewhat  lower  resistance 
than  the  coils  M  and  M' ';  and,  owing  to  the  fact  that  they  have 
a  larger  number  of  turns  and  have  iron  cores  of  larger  size,  they 
have  a  greater  impedance  and  are  known  as  impedance  coils. 
The  direct  current  of  the  line,  therefore,  will  flow  very  readily 
through  the  two  long  coils  and  will  magnetize  the  cores,  giving 
the  effect  of  a  permanent  magnet,  but  on  account  of  the  high 
impedance  of  these  coils,  the  high-frequency  voice  currents  will 
flow  more  readily  through  the  coils  M  and  M',  and  will  affect 
the  magnetic  strength  of  the  cores  exactly  as  do  the  coils  of  the 
polarized  receiver. 

91.  Self-induction. — A  most  important   property  of  electric 
circuits  is  their  action  and  reaction  upon  each  other.     Fig.  15 
shows  that  every  current  carrying  wire  is  surrounded  by  a  mag- 
netic field.     The  current  in  the  wire  builds  up  this  field.     It  has 


FIG.  37. 


58  PRINCIPLES  OF  THE  TELEPHONE 

also  been  shown  that  whenever  the  intensity  of  a  magnetic  field 
around  a  conductor  changes  either  by  being  built  up,  by  decay, 
or  by  relative  motion  in  such  a  way  that  the  magnetic  lines  cut 
across  the  conductor,  an  electromotive  force  is  developed  in  the 
conductor.  This  electromotive  force  is  in  such  a  direction  as  to 
oppose  any  change  in  the  current  flowing  or  in  the  magnetic 
field  surrounding  the  conductor.  This  principle  of  electro- 
magnetic induction  evidently  does  not  depend  upon  the  source 
of  the  magnetic  field.  Hence  an  e.m.f.  is  induced  in  a  conductor 
when  the  current  in  the  conductor  changes,  for  every  change  in 
current  is  accompanied  by  a  change  in  the  density  of  the  mag- 
netic field  which  is  due  to  the  current.  This  principle  of  in- 
ducing an  e.m.f.  by  variations  in  the  current  flowing  is  known  as 
self-induction. 

When  an  e.m.f.  is  impressed  upon  a  circuit,  the  current  can 
not  rise  to  a  maximum  value  at  once  on  account  of  the  counter- 
pressure  of  self-induction. 

It  is  very  evident  that  the  value  of  the  counter  e.m.f.  of  self- 
induction  depends  upon  the  rate  at  which  the  flux  surrounding 
the  conductor  changes;  hence  it  will  depend  upon  the  shape  of  the 
circuit.  If  the  conductor  is  wound  into  a  coil  of  such  shape  that 
all  of  the  magnetic  lines  thread  through  it,  the  counter-pressure 
of  self-induction  for  a  corresponding  change  in  the  current  will 
be  greater  than  when  the  conductor  is  straight. 

92.  Self-inductance. — If  a  conductor  is  wound  into  a  coil  so 
that  if  the  current  varies  at  the  rate  of  1  amp.  per  second  the 
pressure  of  self-induction  is  1  volt,  the  coil  is  said  to  have  unit 
self -inductance.     Self-inductance   is   thus   the   numerical   value 
of  the  property  which  causes  a  counter-pressure  of  self-induction 
to  be  developed.     It  may  also  be  defined  as  the  ratio  of  the  flux 
threading  through  a  coil  to  the  current  producing  it.     For  any 
given  coil  with  an  air  core  the  self-inductance  is  a  constant 
quantity.     If  the  coil  has  an  iron  core  the  self-inductance  is  not 
constant  for  the  reason  that  the  flux  does  not  vary  uniformly 
with  the  current.     In  other  words,  the  permeability  of  iron  is 
not  constant,  but  varies  with  the  current. 

93.  Mutual  Induction. — If  the  magnetic  flux  produced  by  a 
current  in  one  coil  threads  through  a  neighboring  coil,  an  electro- 
motive force  is  also  induced  in  the  second  coil.     This  e.m.f.  is 
said  to  be  due  to  mutual  induction.     In  so  far  as  the  physical 
principles  are  concerned,  self  and  mutual  induction  are  alike. 


RECEIVERS  AND  INDUCTION  COILS  59 

The  only  difference  is  that  in  one  case  the  e.m.f.  is  induced  in  the 
circuit  in  which  the  current  is  flowing,  and  in  the  other  case  the 
e.m.f.  is  induced  in  a  neighboring  but  separate  circuit. 

94.  Impedance. — In  any  circuit  that  has  self-induction  any 
change  in  the  current  will  be  opposed.     When  the  current  is 
increasing  the  induced  e.m.f.  opposes  the  increase  and  when  the 
current  is  decreasing  the  e.m.f.  induced  is  in  such  a  direction  as 
to  oppose  the  decrease.     The  value  of  this  opposing  e.m.f.  de- 
pends not  only  upon  the  self-inductance  of  the  circuit  or  coil, 
but  also  the  rate  at  which  the  current  is  changing. 

If  an  alternating  e.m.f.  of  a  given  value  is  impressed  upon  a 
coil  which  has  some  self-inductance,  the  resulting  current  will  be 
smaller  than  if  a  constant  direct  e.m.f.  were  impressed  upon  the 
same  coil.  The  ratio  of  the  e.m.f.  to  the  current  flowing  is 
called  the  impedance.  In  other  words,  the  alternating  e.m.f. 
divided  by  the  impedance  gives  the  current  in  the  circuit.  The 
higher  the  impedance,  the  smaller  the  current.  As  the  im- 
pedance increases  with  the  frequency,  the  higher  the  frequency 
the  smaller  the  current  in  an  inductive  circuit.  The  currents 
in  a  telephone  line  which  are  produced  by  the  voice  are  of  high 
frequency,  hence  only  a  small  current  will  pass  through  a  coil 
which  has  considerable  inductance. 

95.  The    Induction    Coil. — In    order    to    transmit    telephone 
messages  to  any  considerable  distance  it  is  necessary  to  use 


FIG.  38a. 

induction  coils  in  the  circuit  to  increase  the  voltage,  so  that 
the  resistance  of  the  line  may  be  overcome. 

The  induction  coil  is  designed  to  raise  the  voltage  of  the 
line  current,  which  is  controlled  by  the  transmitter.  The  current 
through  the  transmitter  and  battery  is  a  direct  current  but  is 
pulsating,  due  to  the  variable  resistance  of  the  transmitter. 
Another  function  of  the  induction  coil  is  to  change  this  pulsating 
direct  current  to  an  alternating  one.  An  induction  coils  are 
shown  in  Figs.  38a  and  386. 

The  induction  coil  consists  of  an  iron  core  and  two  windings 


60 


PRINCIPLES  OF  THE  TELEPHONE 


of  insulated  wire,  known  as  the  primary  and  secondary,  as  shown 
in  the  diagram  of  Fig.  39.  The  core  is  made  of  a  bundle  of  iron 
wires  which  have  been  softened  by  annealing.  The  primary 
winding  consists  of  from  250  to  600  turns  of  insulated  copper 
wire,  and  the  secondary  of  from  1,500  to  3,500  turns.  The 
primary  is  made  of  larger  wire  than  the  secondary,  and  the  two 
windings  are  not  connected  inside  the  coil.  The  primary  is 
connected  in  the  circuit  with  the  transmitter  and  batteries,  as 


TIG.  386. 

shown  in  Fig.  40,  and  the  secondaries  are  connected  to  the  line. 
Hence  the  only  currents  which  flow  in  the  line  are  the  alternat- 
ing currents  induced  in  the  secondaries  of  the  coils. 

The  operation  of  the  induction  coil  is  as  follows:  The  pul- 
sating current  flowing  through  the  primary  induces  a  pulsating 
magnetic  field  in  the  iron  core,  increasing  as  the  current  increases 
and  decreasing  as  the  current  decreases.  The  secondary,  being 
wound  on  the  same  core  as  the  primary,  must  cut  the  magnetic 


FIG.  39. 

lines  every  time  the  field  changes.  Thus  a  voltage  will  be  in- 
duced in  the  secondary  winding  every  time  there  is  a  change  in 
the  value  of  the  core  magnetism.  When  this  field  increases 
there  will  be  a  voltage  induced  in  one  direction,  and  when  it 
decreases  a  voltage  will  be  induced  in  the  opposite  direction. 
An  alternating  current  thus  flows  in  the  line.  Since  the  mag- 
netic field  is  of  the  same  value  through  each  of  the  turns  of 
both  the  primary  and  secondary,  at  any  instant  the  voltage  of 
one  turn  of  the  primary  which  causes  the  change  of  field  must 


RECEIVERS  AND  INDUCTION  COILS  61 

equal  the  voltage  caused  by  that  field  in  one  turn  of  the  second- 
ary. Since  the  turns  are  all  in  series,  the  primary  and  secondary 
voltages  will  be  in  the  same  ratio  as  the  number  of  turns  in 
the  primary  and  secondary  windings.  Thus  for  a  coil  having 
500  primary 'turns  and  2,000  secondary  turns,  the  voltage  ratio 
would  be  1  to  4;  and  if  the  primary  pressure  were  3  volts,  the 
secondary  would  be  12  volts. 

The  induction  coil  helps  transmission  by  permitting  the  use 
of  a  low-resistance  battery  circuit;  by  producing  alternating 
currents ;  and  by  producing  currents  of  higher  voltage  than  could 
be  successfully  handled  by  a  transmitter. 


0^3= 


* 

t 


FIG.  40. 

QUESTIONS 

1.  Explain  the  action  of  a  telephone  receiver. 

2.  Why  is  a  permanent  magnet  used  instead  of  a  soft-iron  core?     Give 
two  reasons. 

3.  Explain  how  a  pressure  can  be  generated  by  induction. 

4.  What  is  a  direct  current?     How  is  it  produced? 

6.  What  is  an  alternating  current?     How  is  it  produced? 

6.  Are  most  induced  currents  direct  or  alternating?     Why? 

7.  Can  a  receiver  be  used  as  a  transmitter?     If  so,  explain  its  action 
as  such. 

8.  What  are  the  objections  to  the  use  of  a  single-pole  receiver? 

9.  Explain  the  construction  of  a  double-pole  receiver. 

10.  Upon  what  does  the  sensitiveness  of  a  receiver  depend? 

11.  Why  are  receivers  not  made  as  sensitive  as  possible? 

12.  What  are  the  advantages  of  inside  binding  posts? 

13.  Why  should  the  magnet  be  attached  to  the  case  near  the  diaphragm? 

14.  For  what  purpose  is  an  induction  coil  placed  in  a  telephone  circuit? 

15.  What  are  the  principal  parts  of  an  induction  coil? 

16.  Explain  how  an  induction  coil  increases  the  voltage  in  a  telephone 
system. 

17.  What  determines  the  ratio  of  the  primary  voltage  to  secondary 
voltage? 


62  PRINCIPLES  OF  THE  TELEPHONE 

18.  In  what  ways  does  an  induction-coil  help  transmission? 

19.  Why  is  an  alternating  current  better  than  a  pulsating  direct  current 
in  the  operation  of  a  receiver? 

20.  Explain  the  operation  of  the  Monarch  direct-current  receiver. 

21.  Explain  self-induction;  mutual  induction.     Cross  talk  is  usually  due 
to  mutual  induction. 

22.  What  is  the  self -inductance  of  a  circuit  if  a  pressure  of  2  volts  is 
induced  when  the  current  in  the  circuit  varies  at  the  rate  of  50  amp.  per 
second?     The  unit  of  self-inductance  is  called  a  henry. 

23.  What  quantities  determine  the  current  strength  when  an  alternating 
pressure  is  connected  to  a  circuit? 


CHAPTER  VII 
SIGNALLING  APPARATUS  AND  CIRCUITS 

96.  Signalling  Circuits. — So  far,   only  the  transmitting  and 
receiving  circuits  (or,  as  they  are  generally  called,  the  talking 
and  listening  circuits)   have  been  considered.     In  addition  to 
these  circuits,  some  means  must  be  provided  for  signalling  the 
subscriber  when  he  is  wanted,  and  likewise  for  him  to  signal  the 
central  operator  when  he  wants  some  other  party. 

In  the  local  battery  system,  that  is,  the  system  where  each 
telephone  instrument  has  its  own  batteries,  a  hand  generator 
or  small  dynamo  and  a  bell  or  ringer  form  the  signalling  circuit. 

97.  Generators. — In  the  study  of  magnetism  it  was  said  that 
if  a  conductor  be  moved  in  a  magnetic  field,  so  as  to  cut  the 
magnetic    flux,    an    electric    pressure  is  generated,   and  if  the 
circuit   be   closed   a  current  will  flow.     The  operation  of  the 
electric  generator  is  dependent  upon  this  principle. 

Faraday,  a  noted  scientist,  discovered  this  principle  of  magneto- 
electric  induction  in  1831.  Following  out  his  experiments  along 
this  line,  he  made  the  first  dynamo  known.  This  model  consisted 
of  a  disk  of  copper  12  in.  in  diameter,  which  was  mounted  on  a 
shaft  so  that  it  was  free  to  rotate  with  the  shaft.  A  permanent 
magnet  was  so  placed  that  its  poles  embraced  the  disk  and  the 
magnetic  lines  of  the  permanent  magnet  passed  through  the  disk. 
When  this  disk  was  caused  to  rotate  between  the  poles  of  the 
magnet,  the  magnetic  lines  were  cut  and  an  electrical  pressure 
was  induced. 

Since  Faraday's  time  the  dynamo  has  been  developed  through 
numerous  types  and  with  many  modifications  of  designs,  but 
all  have  been  developed  on  the  principle  which  he  laid  down  and 
which  has  been  stated  as  follows: 

When  a  conductor  is  moved  in  a  magnetic  field  so  as  to  cut 
the  magnetic  lines,  there  is  an  electromotive  force  induced  in 
the  conductor,  in  a  direction  at  right  angles  to  the  direction  of 
motion  and  also  at  right  angles  to'  the  direction  of  the  magnetic 
lines. 

While  the  Faraday  disk  dynamo  did  not  generate  anyj3on- 
7  63 


64  PRINCIPLES  OF  THE  TELEPHONE 

siderable  pressure,  it  led  the  way  to  the  development  of  other 
forms  of  the  dynamo,  in  which  the  arrangement  of  the  conductors 
with  respect  to  the  field  was  better  suited  to  the  development  of 
large  pressures  and  currents. 

The  necessary  elements  of  a  dynamo  for  the  generation  of 
current  are  a  magnetic  field  and  a  conductor  or  conductors  which 
can  be  caused  to  move  across  the  field.  The  simplest  form  of 
such  a  machine  is  shown  in  Fig.  41.  A  loop  of  wire  is  mounted 
on  a  shaft  so  that  it  can  be  rotated  on  its  axis  and  cut  through 
the  magnetic  lines  passing  from  N.  to  S.  At  the  instant  at  which 
the  loop  is  in  the  vertical  position,  as  shown  by  the  full  lines,  the 
^_^  conductors  are  moving  paral- 

lel to  the  magnetic  lines,  and 
since  there  are  no  lines  being 
cut,  no  pressure  is  induced. 

When  the  loop  is  in  the  hori- 
zontal position,  as  shown  by 
the  dotted  lines,  it  is  moving 


FIG  41  o  ne  mag- 

netic lines;  and  as  the  rate  of 

cutting  magnetic  lines  is  a  maximum  in  this  position,  the  pres- 
sure induced  will  be  a  maximum. 

The  direction  of  the  pressure  induced,  when  the  conductor 
is  passing  through  the  field  in  this  direction  of  rotation,  is  from 
front  to  back  on  the  right  side  of  the  loop,  and  from  back  to  front 
on  the  left  side  of  the  loop.  The  pressures  in  the  two  sides  of 
the  loop  will  tend  to  cause  current  to  flow  in  the  same  direction 
around  the  loop  and  the  pressure  in  the  loop  will  be  the  sum  of 
the  pressures  induced  in  the  two  conductors. 

When  the  loop  is  moved  90  degrees  further  it  will  again  be  in 
a  vertical  position,  but  the  conductor  which  was  at  the  top  in 
the  first  case  will  now  be  at  the  bottom,  and  the  other  will  be  at 
the  top.  Advancing  the  loop  past  this  point,  a  pressure  will 
again  be  induced  in  the  conductor,  but  the  position  of  each  con- 
ductor with  respect  to  the  N.  and  S.  poles  will  be  reversed.  Since 
the  positions  of  the  conductors  have  been  interchanged  and  the 
current  has  the  same  direction  with  respect  to  the  magnetic  field 
and  the  direction  of  motion,  it  will  flow  through  the  loop  in  a 
direction  opposite  to  that  of  the  first  half  revolution.  That  is, 
the  direction  of  the  flow  of  current  through  the  loop  will  reverse 
every  180  degrees. 


SIGNALLING  APPARATUS  AND  CIRCUITS        65 

If  two  rings  be  attached  to  the  loop,  one  being  connected 
to  each  end,  and  mounted  so  they  will  rotate  with  the  shaft 
and  loop,  the  pressure  at  these  rings  will  reverse  each  time 
the  loop  has  passed  through  180  degrees.  Current  can  be  taken 
from  these  rings  by  placing  a  brush  on  each  of  the  rings,  as 
shown  in  Fig.  42  and  connecting  them  to  a  conducting  circuit. 


FIG.  42. 

Since  the  direction  of  the  induced  pressure  changes  each 
180  degrees,  the  flow  of  current  in  the  circuit  will  reverse  corj 
respondingly.  That  is,  an  alternating  current  will  flow  in  the 
circuit.  This  changing  of  current  from  zero  to  a  maximum 
value,  then  decreasing  to  zero,  reversing,  building  up  to  a 
maximum  value  in  the  opposite  direction,  and  then  decreasing  to 


FIG.  43. 

zero  again,  is  shown  by  Fig.  43  and  is  known  as  a  cycle.  The 
number  of  cycles  per  second  is  called  the  frequency.  When  the 
loop  is  in  the  vertical  position  as  shown  by  full  lines  in  Fig.  41, 
the  pressure  is  zero  corresponding  to  point  A,  or  zero  degrees, 
Fig.  43.  The  curve  above  the  line  ABC  shows  positive  voltages, 
and  that  below  shows  negative  voltages. 


66 


PRINCIPLES  OF  THE  TELEPHONE 


If  this  generator  be  supplied  with  a  commutator  as  shown 
in  Fig.  44,  the  output  will  be  a  pulsating  direct  current.  It 
has  been  shown  above  that  the  current  in  the  armature  coil 
reverses  every  time  the  conductors  pass  from  one  pole  to  the 
next,  and  that  an  alternating  current  flows  in  the  coil.  HOW- 


FIG.  44. 

ever,  the  coil  is  so  connected  to  the  commutator  that  every 
time  a  conductor  passes  through  a  neutral  point,  the  brush 
contact  is  changed  from  one  segment  to  the  next;  thus  the 
current  through  the  brushes  is  always  in  the  same  direction. 

The  curve  in  Fig.  45  shows  the  effect  the  commutator  has  on 
the  alternating  voltage  represented  in  Fig.  43. 


90° 


360' 


FIG.  45. 


98.  The  Telephone  Generator. — The  telephone  generator  or 
magneto  is  usually  an  alternating-current  generator,  one  form 
of  which  is  shown  in  Fig.  46. 

The  magnetic  field  of  such  a  generator  is  produced  by  U-- 
shaped permanent  magnets.  In  the  figure,  five  permanent 
magnets  are  shown,  but  the  number  used  varies  from  two  to 
six  in  different  classes  of  service,  depending  upon  the  ringing 
power  required.  Several  small  magnets  instead  of  one  large 
one  are  used,  because  it  is  easier  to  properly  shape  the  smaller 
ones,  and  also  because  small  pieces  of  steel  can  be  magnetized 
more  readily  than  large  ones.  .  It  is  necessary  that  the  permanent 
magnets  retain  their  strength  indefinitely,  if  the  generator  is 


SIGNALLING  APPARATUS  AND  CIRCUITS        67 

to  remain  in  service.  Cast-iron  pole  pieces  are  usually  attached 
to  the  permanent  magnets  in  order  to  have  the  pole  face  conform 
to  the  shape  of  the  armature,  and  thus  have  a  small  air  gap  for 
the  magnetic  lines  to  cross.  The  pole  pieces  are  also  made  of 
use  in  holding  the  magnets  together,  and  are  usually  connected 
with  each  other  by  brass  rods  which  hold  them  the  proper  dis- 
tance apart.  It  is  very  necessary  that  this  adjustment  be  main- 
tained, as  there  is  little  clearance  between  the  pole  faces  and  the 
armature.  If  it  ever  becomes  necessary  to  take  a  magneto- 
generator  apart,  care  should  be  taken  in  assembling  the  magnets 
so  as  to  place  all  like  poles  together.  In  case  of  the  reversal  of 


FIG.  46. 

one  magnet  in  a  five-bar  generator,   only  three  bars  will  be 
effective. 

The  armature  consists  of  a  large  number  of  turns  of  fine 
insulated  copper  wire  wound  on  an  iron  core.  A  common  form 
of  core  shown  in  Fig.  47  is  of  cast  iron  or  is  built  up  of  thin, 
soft-iron  sheets  having  the  shape  shown  in  the  end  view  of  the 
armature.  If  built  up,  enough  sheets  to  form  a  core  of  the 
desired  length  are  riveted  together.  Inside  the  hollow  shaft,  S, 
is  a  brass  rod,  P,  which  is  insulated  from  the  shaft  by  a  fiber 
sleeve,  F.  One  end  of  the  coil  is  connected  to  the  insulated 
brass  rod  and  the  other  end  is  connected  to  the  armature  core, 
and  thus  through  the  armature  shaft  and  bearings  is  connected 


68 


PRINCIPLES  OF  THE  TELEPHONE 


to  the  frame  of  the  machine.  The  bearings  for  the  armature 
shaft  are  of  brass  and  are  attached  to  the  pole  pieces.  On  one 
end  of  the  armature  shaft  is  a  pinion  driven  by  a  larger  gear 
when  the  crank  is  turned. 

Another  form  of  armature  known  as  the  H-type  is  shown 
in  Fig.  48.  The  core  is  held  between  two  end  plates,  B  and 
B',  having  projecting  studs,  S  and  S',  which  take  the  place  of 


FIG.  47. 

the  shaft.  The  main  advantage  of  this  type  of  armature  is 
that  the  winding  space  is  large,  as  it  is  not  obstructed  by 
the  shaft,  and  will  accommodate  a  large  number  of  turns.  One 
end  of  the  coil  is  connected  to  the  core,  and  the  other  to  the 
insulated  pin,  P,  which  passes  through  the  stud,  S.  This 
form  of  armature  is  used  in  the  Kellogg  generator  shown  in 
Fig.  46. 


FIG.  48. 

99.  Automatic  Switch. — Every  telephone  generator  is  provided 
with  an  automatic  switch  which  disconnects  the  generator  from 
the  line  when  it  is  not  in  use.  Such  a  switch  is  operated  by  a 
lengthwise  movement  of  the  main  generator  shaft.  In  early 
types  of  telephones  the  required  lengthwise  movement  of  the 
shaft  was  produced  by  the  subscriber's  pushing  in  on  the  crank 
while  ringing.  In  modern  telephones  the  switch  is  operated  auto- 
matically when  the  subscriber  first  turns  the  crank  by  the  use  of 
some  device  similar  to  that  shown  in  Fig.  49.  In  the  device 
shown  the  shaft  can  be  moved  within  the  hub  of  the  large 
gear.  The  spring,  S,  between  the  hub  of  the  gear  and  the  collar, 
C,  on  the  shaft  holds  the  crank  to  the  right  when  it  is  not  in  use. 


SIGNALLING  APPARATUS  AND  CIRCUITS        69 

However,  as  soon  as  the  crank  is  turned,  the  notch,  F,  in  the 
collar,  B,  forces  the  shaft  to  the  left  before  the  large  gear  com- 
mences to  turn,  and  operates  the  switch;  since  the  lengthwise 
movement  of  the  shaft  takes  place  under  less  force  than  is 
required  to  cause  the  armature  to  turn.  The  distance  which 
the[shaft  moves  to  the  left  is  determined  by  the  distance  between 
the  hub  of  the  gear  and  the  collar,  C. 

Local  battery  telephones  may  be  divided  into  two  general 
classes;  series  and  bridging.  The  automatic  generator  switch 
used  must  be  different  in  the  two  cases.  These  differences 
are  taken  up  in  a  later  chapter  devoted  to  these  two  classes 
of  instruments. 


FIG.  49. 

100.  The  Ringer. — The  bell  used  in  telephone  signalling  is 
ordinarily  known  as  a  polarized  bell  or  ringer  because  the  moving 
part  or  armature  is  permanently  magnetized  or  polarized. 

In  Fig.  50  is  shown  a  common  form  of  telephone  ringer. 
M  and  M '  are  the  coils  composed  of  a  large  number  of  turns  of 
small  wire  wound  upon  soft-iron  cores,  both  cores  being  at- 
tached to  the  iron  yoke,  Y.  The  iron  armature,  A,  is  pivoted 
at  F,  and  has  the  clapper  rod,  C,  attached  to  its  center;  hence 
any  movement  of  the  armature  results  in  a  movement  of  the 
clapper.  The  armature  is  supported  by  the  brass  bar,  B,  which 
can  be  adjusted  to  vary  the  distance  between  the  poles  of  the 
electromagnet  and  the  armature.  The  permanent  magnet  is 
not  in  contact  with  the  armature;  hence  the  armature  is  in- 
fluenced by  the  permanent  magnet  by  induction,  and  since 
the  N.  pole  of  the  permanent  magnet  is  opposite  the  center 
of  the  armature,  a  S.  pole  will  be  induced  at  that  point  and 
the  two  ends  of  the  armature  will  become  N.  poles,  exactly  as  if 
the  armature  were  a  part  of  the  permanent  magnet. 


70 


PRINCIPLES  OF  THE  TELEPHONE 


When  no  current  is  flowing  in  the  coils,  either  end  of  the 
armature  will  attract  either  magnet  core,  as  the  armature  is 
polarized  by  the  permanent  magnet. 

When  current  flows  through  the  winding  of  an  electromagnet, 
one  core  becomes  a  S.  pole,  and  the  other  becomes  a  N.  pole. 
Hence  in  the  ringer  shown  in  the  figure  current  flowing  in  one 
direction  through  the  coils  will  cause  the  core  M  to  be  a  N.  pole 
and  the  core  M'  to  be  a  S.  pole.  Since  the  armature  has  a  N.  pole 
at  each  end,  one  end  of  the  armature  will  be  attracted  by  the  S. 
pole  of  the  electromagnet,  and  the  other  will  be  repelled  by  the 


N.  pole  of  the  electromagnet,  and  the  armature  will  be  tilted, 
causing  the  clapper  to  move  to  one  side.  If  the  current  in  the 
coils  be  reversed,  the  polarity  of  the  magnet  cores  will  be  reversed, 
causing  the  armature  to  be  tilted  in  the  opposite  direction. 

When  alternating  current  is  used  in  ringing,  the  polarity  of  the 
magnet  cores  reverses  just  as  often  as  the  current  in  the  coils 
reverses  in  direction.  Hence  every  time  the  current  reverses 
the  armature  is  tilted  from  one  side  to  the  other,  causing  the 
ball  of  the  clapper  to  strike  one  of  the  gongs.  The  rapidity  of 
ringing  depends  upon  the  frequency  or  rate  of  reversal  of  the 
alternating  current.  As  the  quality  of  the  ring  depends  largely 


SIGNALLING  APPARATUS  AND  CIRCUITS        71 

upon  the  position  of  the  gongs,  they  are  made  adjustable  with 
reference  to  the  ball  of  the  clapper. 

The  Kellogg  ringer  has  no  provision  for  adjustment  of  the 
distance  between  the  armature  and  magnet  poles  after  leaving  the 
factory.  As  this  adjustment  is  very  close,  a  strip  of  German 
silver  is  placed  between  the  armature  and  magnet  poles  to  keep 
them  from  coming  into  direct  contact  and  thus  freezing  or  stick- 
ing together.  The  gongs  are  adjustable  as  in  the  ringer  described 
above. 


CHAPTER  VIII 
THE  SUBSCRIBER'S  TELEPHONE  SET 

101.  The  Complete  Telephone. — The  following  separate  pieces 
of  telephone  apparatus  have  been  discussed :     The  battery,  trans- 
mitter, induction  coil,  receiver,  generator,  and  ringer.     These 
parts  make  up  the  talking  and  signalling  circuits,  and  in  order  to 
have  successful  commercial  operation  must  be  connected  in  a 
certain  manner. 

It  is  necessary  to  save  battery  current  when  the  transmitter 
is  not  in  use;  hence  some  means  must  be  provided  for  opening 
the  battery  circuit  when  the  instrument  is  idle,  since  the  trans- 
mitter itself  never  opens  this  circuit,  but  merely  changes  or 
controls  the  resistance  of  the  same. 

The  receiver  must  also  be  disconnected  from  the  line  when 
not  in  use,  so  as  to  leave  the  line  free  for  signalling  currents. 

The  signalling  circuit  must  be  disconnected  when  the  talking 
circuit  is  being  used. 

102.  The  Hook  Switch. — The  above  connections  are  estab- 
lished and  broken  by  the  hook  switch.     In  order  to  make  the 
action  of  the  hook  switch  as  nearly  automatic  as  possible,  the 
hook  is  made  the  most   convenient   point  on  which  to  hang 
the  receiver  when  it  is  not  in  use,  as  it  is  not  rigidly  attached  to 
the  telephone.     When  the  receiver  is  on  the  hook,  its  weight  pulls 
the  hook  down  and  holds  the  battery  or  transmitting  circuit 
open,  thus  saving  battery  current,  and  also  holds  the  receiver 
circuit  open,  leaving  the  line  free  for  signalling  purposes.     When 
the  receiver  is  off  the  hook,  however,  a  spring  raises  the  latter, 
and  closes  the  circuits  through  the  receiver  and  transmitter  so 
that  the  line  can  be  used  for  communication. 

The  Western  Electric  hook  switch  for  wall  telephones,  shown 
in  Fig.  51,  is  commonly  known  as  the  short-lever  type.  The 
hook  is  pivoted  to  the  bracket,  and  when  the  receiver  is  re- 
moved the  lever  is  raised  by  the  hook  spring  which  at  the  same 
time  closes  the  switch  contacts.  When  the  receiver  is  hung  on 
the  hook,  its  weight  pulls  the  lever  down  and  compresses  the 

72 


THE  SUBSCRIBER'S  TELEPHONE  SET 


73 


hook  spring,  allowing  the  switch  springs  to  fall  back  against  the 
two  short  springs  which  act  only  as  supports.  These  springs 
are  so  spaced  that  when  the  lever  is  clear  down,  all  the  contacts 
shown  will  be  open.  The  spring  arrangement  shown  is  varied 
to  suit  the  conditions  under  which  the  hook  is  used. 

The  Stromberg-Carlson  hook  switch,  shown  in  Fig.  52,  oper- 
ates on  a  somewhat  different  principle.     The  bracket,  F}  on 


FIG.  51. 


FIG.  52. 


which  the  hook  and  switches  are  mounted,  is  a  one-piece  steel 
punching.  The  spring,  S,  raises  the  hook  when  the  receiver  is 
in  use.  When  the  receiver  is  placed  on  the  hook  its  weight 
pulls  it  down,  and  the  rubber  roller,  P,  forces  apart  the  springs 
A  and  D,  thus  opening  the  contacts.  The  hook  can  be  re- 
moved from  the  telephone  cabinet  for  shipping  or  transportation 


FIG.  53. 

purposes  if  desired  without  disturbing  the  adjustment  of  the 
switch  springs. 

The  Kellogg  hook  switch,  shown  in  Fig.  53,  is  of  the  long- 
lever  type,  having  all  parts  mounted  on  the  casting,  F,  which 
is  firmly  secured  to  the  backboard  of  the  telephone.  The  switch 
springs  are  operated  by  means  of  the  fiber  roller,  P.  In  the 
switch  shown,  when  the  hook  is  up  the  two  upper  contacts  be- 
tween A,  B,  and  C  are  open.  When  the  hook  is  down  this  con. 


74 


PRINCIPLES  OF  THE  TELEPHONE 


dition  is  reversed  and  the  lower  contacts  are  closed.  These 
switch  springs  are  of  German  silver,  having  platinum  contact 
points.  Platinum  points  are  used  in  hook  switches  because  this 
metal  does  not  corrode  readily  and  the  contacts  are  therefore 
easy  to  maintain. 

Hook  switches  for  use  in  desk  stands  are  of  somewhat  different 
design.  The  Western  Electric  switch  is  contained  in  the  barrel 
of  the  stand,  and  is  operated  by  the  lever  very  much  as  in  the 
wall  telephone  of  the  same  make. 

The  Kellogg  desk  stand  hook  switch  is  arranged  somewhat 
differently,  as  is  shown  in  Fig.  54,  the  switch  springs  being  placed 


FIG.  54. 

in  the  base  of  the  instrument.  The  hook  lever  is  in  two  parts. 
One  part  is  pivoted  at  P,  and  the  other  at'D  to  the  spring  B; 
and  the  two  points  are  pinned  together  at  C.  When  the  hook  is 
raised  the  spring  contact  is  closed.  When  the  lever  is  depressed 
the  point  C  is  moved  to  the  right  and  D  is  forced  downward, 
opening  the  contact  between  A  and  B.  Other  spring  arrange- 
ments are  used  for  different  classes  of  service,  as  mentioned  above. 

QUESTIONS 

1.  Of  what  does  the  signalling  circuit  of  a  telephone  consist?     Show 
by  diagram. 

2.  For  what  is  the  generator  used?     The  ringer? 


THE  SUBSCRIBER'S  TELEPHONE  SET  75 

3.  Upon  what  principles  does  the  operation  of  a  dynamo  depend? 

4.  In  the  simple  dynamo  shown  in  Fig.  41  why  do  not  the  voltages 
generated  in  each  side  of  the  loop  oppose  each  other? 

5.  What  kind  of  a  current  will  be  generated  by  the  simple  dynamo 
mentioned  above?     Explain. 

6.  Of  what  use  is  a  commutator?     Are  magneto  generators  ever  provided 
with  a  commutator? 

7.  How  is  the  magnetic  field  of  a  telephone  generator  constructed? 

8.  Describe  two  types  of  magneto  armatures. 

9.  For  what  is  the  automatic  switch  used?     How  does  it  operate? 

10.  Why  is  the  ordinary  telephone  ringer  said  to  be  polarized  ? 

11;  Explain  the  action  of  the  ringer,  including  the  effect  of  the  permanent 
magnet  on  the  armature. 

12.  For  what  is  the  hook  switch  used?     Explain  in  a  general  way  how  the 
hook  switch  works. 

13.  Name  the  parts  of  the  talking  circuit. 

14.  Show  how  the  Kellogg  desk  stand  hook  switch  operates. 


CHAPTER  IX 


LOCAL  BATTERY  SYSTEMS 

103.  Classification  of  Local  Battery  Systems. — It  has  been 
mentioned  previously  that  local  battery  telephones  are  of  two 
classes :  series  and  bridging.     The  series  instruments  are  so  named 
because  the  generator  and  ringer  are  placed  in  series  with  each 
other.     The  bridging  instruments  are  so  called  because  the  bell 
and  generator  are  separately  bridged  or  connected  in  parallel 
across  the  line. 

104.  Series  Telephone  System. — Fig.  55  is  a  diagram  showing 
the  connections  of  the  various  parts  of  the  series  telephone. 


FIG.  55. 

When  the  receiver  is  on  the  hook  the  signalling  circuit  is  closed 
at  the  point  C.  If  the  central  operator  wishes  to  call  the  left 
station  of  Fig.  55,  ringing  current  is  sent  over  the  line,  let  us  say, 
entering  at  terminal  LI,  and  leaving  at  L2  as  follows:  Following 
the  line  from  LI,  we  reach  the  spring,  R,  of  the  generator.  For 
the  operation  of  the  series  telephone  ringer,  a  generator  with  an 
automatic  switch,  as  shown  in  Fig.  49,  is  required.  This  switch 

76 


LOCAL  BATTERY  SYSTEMS  77 

contains  two  springs,  R  and  Q,  insulated  from  each  other  except 
when  the  switch  is  closed.  The  spring  R  is  in  contact  with  the 
insulated  pin,  P,  to  which  one  end  of  the  armature  coil  is  con- 
nected; and  the  spring  Q  is  attached  to  the  frame  of  the  generator, 
to  which  the  other  end  of  the  armature  coil  is  connected.  Hence 
these  two  springs  form  the  terminals  of  the  armature  coil.  One 
of  the  terminals,  either  TI  or  T2,  is  connected  to  the  line,  and 
the  other  terminal  is  connected  to  one  end  of  the  bell  or  ringer 
coil.  When  the  generator  is  not  in  use  the  circuit  is  closed, 
through  the  two  springs,  R  and  Q,  as  shown  in  Fig.  55,  thus 
forming  a  low-resistance  shunt  for  the  generator  and  allowing 
the  ringing  currents  to  flow  readily  through  the  bell.  If  these 
currents  had  to  flow  through  the  high  resistance  of  the  armature 
coil,  in  addition  to  the  bell  coil,  they  would  be  much  weakened. 

As  soon  as  the  subscriber  has  heard  his  bell  ring,  he  removes 
the  receiver  from  the  hook  which  is  raised  by  a  spring,  opening 
the  signalling  circuit,  at  the  same  time  closing  the  talking  and 
listening  circuits  through  the  contact  points,  a  and  6.  The 
diagram  at  the  right  in  Fig.  55  shows  the  connections  after  the 
receiver  has  been  removed  from  the  hook,  the  contacts  a  and  b 
being  closed  and  contact  c  opened. 

Referring  to  the  diagram  at  the  right  of  Fig.  55,  the  talking 
current  can  be  traced  through  the  receiver,  reproducing  the 
sounds  of  the  operator's  voice,  and  through  the  secondary  of 
the  induction  coil  to  the  point  j.  From  j  the  current  flows 
through  the  wire  to  contact  a,  through  the  receiver  hook  and  to 
L2.  From./  to  the  hook  there  are  two  paths  for  the  current,  one 
through  the  wire  connecting  j  and  a,  and  the  other  through  the 
primary  of  the  induction  coil,  the  transmitter,  and  battery;  but 
the  resistance  of  this  latter  circuit  is  so  much  higher  than  that 
of  the  first  that  practically  all  of  the  current  flows  through  the 
wire  directly  from  j  to  a. 

From  the  diagram  of  connections  it  is  evident  that  battery 
current  is  flowing  in  the  transmitter  circuit  when  the  receiver 
hook  is  up,  whether  the  transmitter  is  in  use  or  not,  since  the 
latter  never  opens  the  circuit.  It  is  the  variation  of  current  in 
the  primary  of  the  induction  coil,  however,  which  causes  current 
to  flow  in  the  secondary,  and  a  continuous  direct  current  (no 
matter  what  its  value  may  be)  can  not  produce  current  in  the 
secondary.  Hence  there  is  no  interference  with  the  talking 
current  from  the  central  operator's  instrument. 


78  PRINCIPLES  OF  THE  TELEPHONE 

105.  Local   Battery   Circuit. — The   circuit   made   up   of   the 
battery,  transmitter,  primary  of  the  induction  coil,  and  the  con- 
necting wires  form  what  is  known  as  the  local  battery  circuit. 

When  the  subscriber  talks,  a  variable  current  is  set  up  in  the 
transmitter  circuit  on  account  of  the  variations  in  resistance  of 
the  transmitter.  This  current  flows  from  the  battery,  through 
the  transmitter,  through  the  primary  of  the  induction  coil,  and 
through  the  contact  a,  through  a  part  of  the  hook,  and  through 
contact  6,  back  to  the  battery.  The  variations  in  the  current 
flowing  in  the  primary  of  the  induction  coil  cause  an  alternating 
current  to  be  induced  in  the  secondary  of  the  coil.  This  current 
flows  through  the  circuit  as  outlined  above,  out  over  the  line, 
and  is  finally  converted  by  means  of  the  receiver  at  the  receiving 
station  into  mechanical  energy  of  sound. 

As  soon  as  the  subscriber  is  through  talking  he  hangs  the  re- 
ceiver on  the  hook.  The  hook,  being  pulled  down,  opens  the 
talking  circuit  and  closes  the  signalling  circuit,  thus  leaving  the 
instrument  ready  to  receive  future  signals. 

If  the  subscriber  wishes  to  signal  the  central  operator,  he 
turns  the  generator  crank  while  the  receiver  is  on  the  hook. 
Whenever  the  generator  crank  is  turned,  the  shaft  is  moved 
horizontally  and  the  pressure  of  the  hard-rubber  tip,  L,  Fig.  49, 
opens  the  circuit  by  pressure  on  the  spring  R,  allowing  the  cur- 
rent to  flow  through  the  line.  If  the  contact  between  R  and  Q 
were  not  broken,  the  current  from  the  generator  would  flow 
through  the  shunt  instead  of  the  line. 

106.  The  Bridging  Telephone.— Fig.  56  is  a  diagram  of  the 
connections  of  the  bridging  telephone.     It  will  be  seen  that  the 
ringer  and  generator  are  connected  in  parallel  across  the  line. 
The  generator  is  provided  with  an  automatic  switch  which  opens 
the  circuit  when  the  generator  is  not  in  use,  so  that  no  current 
can  be  shunted  from  the  ringer  or  talking  circuit.     The  switch 
has  three  springs:  R,  Q,  and  /.     The  ends  of  the  armature  coil 
are  connected  to  spring  R  and  to  the  frame  of  the  generator. 
When  the  subscriber  is  called,  the  ringing  current  passes  from 
LI  to  spring  Q,  then  to  spring  J,  through  the  bell  coils  and  to  the 
other  side  of  the  line. 

When  the  subscriber  wishes  to  call  central  or  some  other  sub- 
scriber, the  motion  of  the  ringer  shaft  to  the  left  opens  the  bell 
circuit  between  Q  and  J  and  closes  the  generator  circuit  between 
R  and  /.  The  ringing  current  leaves  through  R  to  Q,  then  to 


LOCAL  BATTERY  SYSTEMS 


79 


line  at  LI  and  returns  by  L2  and  the  connecting  wires  to  the 
generator  frame  to  which  the  other  generator  terminal  is  con- 
nected. The  pin,  P,  is  insulated  from  the  generator  frame. 
Bridging  bell  coils  are  wound  to  a  resistance  of  1,000  to  2,500 
ohms,  varying  with  different  manufacturers. 

The  number  of  turns  on  the  bell  coils  is  much  greater  than  with 
the  series  bells;  therefore  the  current  necessary  for  ringing 
is  much  less  than  with  the  other  type.  Again,  the  impedance 
of  the  bell  coils  is  so  high,  compared  with  that  of  the  circuit 
through  the  telephone  receivers,  that  the  amount  of  high-fre- 


FIG.  56. 

quency  talking  current  shunted  is  of  no  consequence  in  the 
operation  of  the  system. 

107.  Connections  of  Bridging  Telephone. — The  connections 
of  the  bridging  telephone,  Fig.  56,  show  that  there  is  no  bottom 
contact  for  the  hook  switch.  When  the  receiver  is  on  the  hook 
the  only  path  for  current  from  the  line  is  from  LI  through  the 
ringer  to  L2.  When  the  receiver  is  removed  from  the  hook,  the 
contacts  a  and  b  are  closed,  establishing  the  listening  and  talking 
circuits,  which  are  the  same  as  in  the  series  instrument. 

When  the  subscriber  wishes  to  signal  the  central  operator  he 
turns  the  generator  crank,  which  closes  the  contact  between  R 
and  Q  and  connects  the  generator  to  the  line,  and  at  the  same 


80 


PRINCIPLES  OF  THE  TELEPHONE 


time  opens  the  contact  between  Q  and  J  and  thus  disconnects 
the  ringer  so  that  none  of  the  calling  current  is  shunted  from  the 
line. 

108.  Telephone   Instruments. — Telephone   instruments   com- 
monly used  are  of  the  types  known  as  wall  and  desk  sets;  the 
former  so  called  because  they  are  usually  attached  to  the  wall, 
and  the  latter  because  they  are  intended  for  use  on  a  desk  or 
table. 

109.  Standard  Wall  Set. — A  common  form  of  magneto  wall 
set  is  shown  in  Figs.  1  and  2;  and  a  view  of  another  instrument 


FIG.  57a. 


FIG.  576. 


of  the  same  type  but  of  different  make  is  shown  in  Figs.  57a 
and,  576. 

It  is  seen  that  the  essential  parts  are  all  mounted  within  the 
cabinet  or  on  the  outside  where  they  are  most  convenient  for 
the  user.  The  transmitter  is  carried  on  a  hollow  adjustable 
arm,  through  which  the  wires  pass  to  the  interior  of  the  cabinet. 
The  ringer  is  mounted  on  the  inside  of  the  door,  and  as  the 
gongs  are  on  the  outside  it  is  necessary  for  the  clapper  rod  to  pass 
through  a  hole  in  the  door.  The  induction  coil  is  mounted  on 
the  inside  of  the  door,  as  is  the  condenser,  the  use  of  which  in 
magneto  telephones  will  be  explained  in  a  later  section  on  party 
lines.  In  some  other  makes  of  telephones  the  induction  coil  is 
not  mounted  on  the  door,  but  is  placed  in  the  cabinet.  The  hook 


LOCAL  BATTERY  SYSTEMS 


81 


switch  is  of  the  short-lever  type,  the  switch  being  inside  the 
cabinet.     The  magneto  is  mounted  on  a  shelf,  and  has  its  shaft 


FIG.  58. 


FIG.  59. 


extending  through  the  right  side  of  the  box.  The  receiver,  as 
in  all  telephones  of  this  type,  is  connected  with  the  instrument 
by  means  of  a  flexible  cord. 


FIG.  60a. 


FIG.  606. 


In  order  that  connections  between  the  parts  mounted  on  the 
door  and  those  within  the  box  will  not  be  broken  when  the  door  is 
opened  for  inspection  or  other  purposes,  the  hinges  are  made  a 


82  PRINCIPLES  OF  THE  TELEPHONE 

part  of  the  conducting  circuit.  A  special  form  of  spring  joins 
the  two  leaves  of  the  hinges  in  order  that  the  circuits  may  not 
be  opened  through  corrosion  and  wearing  of  the  parts. 

A  wiring  diagram  of  the  series  instrument  of  this  type  is 
shown  in  Fig.  58,  and  a  diagram  of  the  bridging  set  is  shown  in 
Fig.  59.  A  comparison  of  these  diagrams  with  the  simplified 
ones  preceding  will  show  the  connection  of  the  parts  to  be 
practically  the  same. 

Another  wall  set  of  standard  make  is  shown  in  Figs.  60a  and  606. 
The  set  shown  in  the  figure  does  not  contain  a  condenser,  but  a 
place  is  provided  for  one  immediately  under  the  induction  coil. 


FIG.  61. 

The  wiring  of  this  set  differs  in  one  respect  from  that  shown  in 
Fig.  57.  The  wires  are  carried  in  a  conduit  from  the  apparatus 
inside  of  the  box  to  that  on  the  door  and  accordingly  the  hinges 
do  not  form  a  part  of  the  electrical  circuit.  The  variation  of  the 
resistance  in  the  hinge  contacts  is  thus  obviated. 

110.  Hotel  Set. — Figs.  61  and  62  show  a  local  battery  telephone 
of  small  size,  commonly  known  as  the  residence  or  hotel  type.  It 
is,  in  fact,  a  simple  magneto  box  cabinet  containing  all  the  talk- 
ing and  signalling  apparatus  except  the  batteries,  which  are 
placed  in  any  convenient  location  away  from  the  instrument, 
This  telephone  has  all  the  operating  advantages  of  other  types, 


LOCAL  BATTERY  SYSTEMS 


83 


and  is  installed  where  a  larger  cabinet  would  be  objectionable, 
and  where  the  writing  shelf  is  not  necessary.     All  of  the  working 


FIG.  62. 

parts  are  of  standard  size,  and  the  wiring  is  practically  the  same 
as  that  of  the  standard  instrument  previously  shown. 


FIG.  63. 

111.  Desk  Set. — A  desk  set  consists  of  the  desk  stand,  com- 
prising the  transmitter,  receiver,  hook  switch,  and  induction 


84 


PRINCIPLES  OF  THE  TELEPHONE 


coil;  the  desk  box,  containing  the  magneto  and  ringer;  and  the 
battery  box,  although  the  latter  is  often  omitted,  as  the  batteries 
may  be  set  in  an  out-of-the-way  place  where  a  box  is  not  re- 
quired. A  desk  stand  and  desk  box  are  shown  in  Figs.  63  and 
64.  The  wiring  diagram  of  a  desk  set  does  not  differ  materially 
from  a  wall  set  except  that  as  the  apparatus  is  not  all  mounted 


FIG.  64. 

in  one  cabinet,  flexible  conductors  are  used  to  connect  the  vari- 
ous parts.  A  wiring  diagram  for  the  Stromberg  and  Carlson 
desk  set  is  shown  in  Figs.  65  and  66. 

An  apparatus  consisting  of  a  combination  of  receiver  and 
transmitter,  and  known  as  a  hand  telephone  is  shown  in  Fig. 
67.  Although  these  are  not  in  common  use",  They  are  made  in 


LOCAL  BATTERY  SYSTEMS 


85 


two  styles.     The  one  shown  in  the  figure  opens  and  closes  the 
circuits  in  the  usual  way,  that  is,  through  the  medium  of  the  hook 


BATTERIES 


FIG.  65. — Desk  telephone  wired  for  series  operation. 


DRY     BATTERIES 


FIG.  66. — Desk  telephone  wired  for  bridging  operation. 

switch.  Another  make  has  no  hook  switch,  the  ringing  circuit 
being  open  and  the  talking  circuit  closed  by  the  pressure  of  a 
lever  in  the  barrel  of  the  instrument. 


86 


PRINCIPLES  OF  THE  TELEPHONE 
QUESTIONS 


1.  What  is  meant  by  a  local  battery  telephone? 

2.  Explain  the  difference  between  the  two  types  of  local  battery  telephones. 

3.  Trace  the  path  of  current  in  series  telephone:  when  the  operator  is 
signalling  the  subscriber;  when  the  subscriber  is  answering  his  call;  and  when 
the  subscriber  is  signalling  the  central  office.     Use  diagrams. 

4.  What  objections  are  there  to  the  use  of  series  instruments  on  party 
lines? 

6.  What  is  the  difference  between  a  series  and  bridging  ringer? 


FIG.  67. 

6.  Trace  the  circuits  of  the  bridging  telephone  as  you  did  for  the  series 
instrument  in  question  3. 

7.  Why  does  not  the  current  from  the  line  flow  through  the  battery  when 
the  hook  is  up,  since  the  circuit  is  closed? 

8.  Explain  why  the  talking  current  does  not  flow  through  the  ringer, 
instead  of  the  receiver,  in  the  bridging  telephone. 

9.  Explain  the  difference  between  the  generator  switches  used  in  series 
and  bridging  instruments,  and  give  the  reasons  for  the  difference. 

10.  Why  does  not  the  battery  current  flowing  through  the  primary  of  the 
induction  coil  interfere  with  the  talking  current  from  the  line  which  passes 
through  the  secondary? 


CHAPTER  X 
COMMON  BATTERY  TELEPHONES 

112.  General. — The  common  battery  or  central  energy  tele- 
phone system  is  so  named  on  account  of  the  fact  that  the  current 
for  the  operation  of  the  system  is  supplied  from  a  central  or 
common  source  instead  of  from  batteries  at  each  subscriber's 
station.  The  common  source  is  invariably  a  storage  battery1 
located  at  the  central  office. 

The  electrical  characteristics  of  common  battery  substation 
apparatus  are  the  same  as  those  of  the  local  battery  equipment, 
but  on  account  of  the  central  source  of  energy  some  of  the 
apparatus  found  in  local  battery  installation  is  not  used,  and 
some  other  equipment  is  added.  The  design  and  connections 
are  also  modified  and  changed.  The  operation  of  the  induction 
coil  in  connection  with  the  condenser  in  the  common  battery 
system  is  somewhat  the  more  complicated. 

A  common  battery  transmitter  has  a  higher  resistance  than  the 
local  battery  transmitter.  This  is  due  to  the  fact  that  the 
voltage  employed  is  much  higher  in  the  former  than  in  the  latter 
systems.  In  general  exchange  practice  the  higher  voltage  is  not 
needed  for  transmission,  but  for  signalling.  The  resistance  of 
the  line  is  the  same  in  the  two  cases.  Therefore,  to  reduce 
the  current  in  the  talking  circuit,  the  resistance  of  the  various 
parts  is  increased.  The  resistance  of  standard  common  battery 
transmitters  of  various  makes  is  in  the  neighborhood  of  100 
ohms. 

The  common  battery  induction  coil  differs  also  somewhat 
from  the  induction  coil  of  the  local  battery  system.  In  the 
latter  system  the  electrical  pressure  employed  is  comparatively 
low,  approximately  4  volts.  This  is  too  low  for  efficient  trans- 
mission, so  an  induction  coil,  is  used  which  transforms  the  low 
primary  pressure  to  a  secondary  pressure  which  is  sufficiently 
high  to  force  the  current  to  the  other  instrument. 

1  The  storage  battery  will  be  explained  in  connection  with  central  office 
equipment. 

9  87 


88  PRINCIPLES  OF  THE  TELEPHONE 

In  common  battery  systems  the  primary  voltage  is  high  enough 
so  that,  instead  of  using  a  step-up  coil,  the  coil  may  be  entirely 
omitted,  or  one  that  slightly  lowers  the  pressure  may  be  em- 
ployed. Induction  coils,  however,  vary  in  this  respect  con- 
siderably, being  designed  for  the  particular  instrument  circuit  in 
which  they  are  used.  As  a  general  rule  it  is  advisable  never  to 
replace  any  part  of  an  instrument  equipment  with  that  of  an- 
other manufacture  unless  investigation  shows  that  the  appara- 
tus which  it  is  proposed  to  substitute  is  designed  for  that  type 
of  circuit. 

The  added  feature  of  the  common  battery  system  is  the  con- 
denser, the  construction  and  use  of  which  will  be  explained 
in  the  succeeding  paragraphs.  Instead  of  each  instrument 
having  a  hand  generator  for  signalling  the  central  office,  the 
current  for  calling  the  operator,  or  central,  is  supplied  by  the 
common  battery.  The  ringing  generator  is  thus  omitted  from 
the  common  battery  system. 

113.  The  Condenser. — When  an  insulated  electrical  conductor 
is  connected  to  a  battery  or  some  other  source  of  electrical 


FIG.  68. 

pressure,  the  conductor  becomes  charged;  that  is,  a  sufficient 
quantity  of  electricity  flows  into  the  conductor  to  raise  its 
potential  or  pressure  to  that  of  the  battery.  If  a  conductor 
be  connected  to  the  positive  terminal  of  a  battery,  it  becomes 
positively  charged,  and  if  it  be  connected  to  the  negative  ter- 
minal it  becomes  negatively  charged.  A  condenser  is  an  ar- 
rangement of  conducting  plates  which  are  insulated  from  each 
other,  and  therefore  can  be  charged  by  connecting  them  to  a 
source  of  electrical  pressure. 

The  most  simple  form  of  a  condenser  consists  of  two  or  more 
conducting  plates  close  together,  but  separated  by  some  in- 
sulating material  called  the  dielectric.  Fig.  68  is  a  cross- 
section  of  a  simple  condenser  of  this  type.  The  heavy  horizontal 
lines  represent  a  cross-section  of  the  conducting  material,  and 
the  fine  dots  represent  the  insulating  material. 


COMMON  BATTERY  TELEPHONES  89 

Capacity  of  a  Condenser. — The  capacity  of  a  condenser 
is  measured  by  the  quantity  of  electricity  required  to  charge  it 
to  a  difference  of  pressure  of  1  volt.  The  unit  of  capacity  is 
called  the  farad,  a  word  derived  from  the  ^name  Faraday.  A 
condenser  is  said  to  have  a  capacity  of  1  farad  when  a  charge  of 
1  coulomb  raises  its  potential  by  1  volt.  The  farad  is  entirely 
too  large  for  practical  purposes,  and  so  the  microfarad,  which  is 
one-millionth  of  a  farad,  is  used.  Condensers  are  thus  rated  in 
microfarads. 

The  capacity  of  a  condenser  depends  upon  several  factors; 
the  number  and  dimensions  of  the  sheets  of  conducting  material, 
the  material  and  thickness  of  the  dielectric.  The  greater  the 
area  of  the  sheets  of  conducting  material  and  the  thinner  the 
layers  of  insulation,  the  higher  the  capacity. 

Furthermore,  if  the  dielectric  is  paraffined  paper,  the  capacity 
is  1.9  to  2.4  times  as  large  as  it  would  be  if  air  of  the  same  thick- 
ness were  used.  This  property  of  the  insulating  material  upon 
which  the  capacity  of  the  condenser  depends  is  called  the  di- 
electric constant  or  specific  inductive  capacity. 

The  principal  materials  used  in  the  manufacture  of  telephone 
condensers  are  tin-foil,  paper,  and  paraffine. 

The  tin-foil  is  made  from  an  alloy  of  about  90  per  cent,  lead 
and  10  per  cent.  tin.  This  is  rolled  out  until  it  is  very  thin. 
In  the  preparation  of  the  foil,  great  care  is  taken  to  insure  purity 
of  the  product  and  freedom  from  grit,  which  would  puncture 
the  condenser  when  assembled  and  pressed. 

The  paper  employed  is  a  special  grade  of  what  is  commercially 
known  as  rice  paper.  It  is  white  in  color,  very  flexible,  of 
high  tensile  strength,  and  quite  tough.  Condenser  paper  is 
purchased  in  several  thicknesses  varying  from  0.0005  to  0.001 
in.  It  is  put  up  in  rolls  of  different  widths,  depending  upon  the 
finished  dimensions  of  the  condenser  for  which  it  is  intended. 

There  are  two  main  reasons  for  the  use  of  paraffine:  first, 
its  insulating  properties  are  quite  high,  and  thus  it  reinforces  the 
dielectric  strength  of  the  paper;  and  second,  its  dielectric  constant 
is  somewhat  higher  than  that  of  paper  alone.  When  a  dielectric 
with  a  large  constant  is  used,  the  dimensions  of  a  condenser  of  a 
given  capacity  are  less  than  when  the  dielectric  constant  is  small. 
This  constant  for  paraffine  ranges  from  1.9  to  2.4,  depending 
somewhat  upon  the  temperature.  The  capacity  of  a  condenser 
may  be  calculated  by  the  following  formula: 


90 


where 


PRINCIPLES  OF  THE  TELEPHONE 

C  =  884  X  10-10  X  -?-  microfarads 

6 


k  =  dielectric  constant 

S  =  area    of    dielectric    between    conducting    plates    in 

square  centimeters 
t  =  thickness  of  dielectric  in  centimeters 


in-10  = 


10,000,000,000 

EXAMPLE 

A  condenser  is  made  of  501  sheets  of  tin-foil  separated  by  sheets  of  par- 
affined paper  0.007  in.  thick.     The  overlapping  portions  of  the  sheets  of 


FIG.  69. 

tin-foil  are  10  in.  by  10  in.  as  shown  in  Fig.  69.     Calculate  the  capacity  of 
the  condenser.  \ 

Solution 

If  there  are  501  sheets  of  tin-foil,  there  are  500  sheets  of  paraffined  paper 
between  the  sheets  of  tin-foil.     The  total  area  of  these  sheets  of  paper  will  be 


Therefore 


Then 


600  X  10  X  10  X  6.45  =  322,500  sq.  cm. 

S  =  322,500  sq.  cm. 
t  =  0.001  X  2.54  =  0.00254  cm. 
K  =  2.3  about 

C  =  884  X  10-io  x  2-3X500  microfarads 


26  microfarads  nearly. 


The  capacity  of  paper  condensers  varies  greatly  with  the  rate  of  charge 
and  discharge. 

114.  Manufacture  of  Telephone  Condensers.  —  The  process  of 
manufacturing  telephone  condensers  is  well  shown  in  Fig.  70. 


COMMON  BATTERY  TELEPHONES 


91 


The  machine  used  for  the  winding  of  the  condensers  is  usually 
provided  with  six  spindles  arranged  to  carry  the  rolls  of  the  paper 
and  foil  in  the  manner  indicated.  A  collapsible  mandrel  upon 


WINDING 

MANDREL. 


FIG.  70. 


which  the  tin-foil  and  paper  are  wound  is  shown  to  the  right. 
The  first  step  in  the  assembly  of  the  condenser  is  the  winding  of 
a  few  turns  of  paper,  only,  on  the  mandrel  to  form  a  core.  This 
is  done  to  avoid  sharp  bends 
in  the  inner  layers  of  the  foil. 
Very  thin  strips  of  brass 
about  Y±  in.  wide  and  about 
an  inch  longer  than  the 
width  of  the  foil  are  at- 
tached to  each  strip  of  foil. 
These  brass  strips  are  used 
to  connect  the  tin-foil  to  the 
terminals  on  the  condenser 
case.  In  some  makes  of 
condensers  the  connecting 
strips  are  placed  midway  in 
the  foil  strip  to  reduce  the 
plate  resistance  and  thus  de- 
crease the  loss  of  energy,  and 
heating  of  the  condenser. 
The  required  number  of 
turns  of  paper  and  foil  are  F 

then  wound  on  the  mandrel, 
the  foil  is  cut  off  and  a  few  extra  layers  of  paper  wound  on  for 
protection. 

The  condensers,  after  being  assembled  in  pressing  "jigs," 
are  next  placed  in  perforated  baskets  and  immersed  in  a  large 
tank  of  molten  paraffine,  a  cover  is  placed  on  the  tank  and  the 
air  exhausted  until  a  desired  degree  of  vacuum  is  obtained. 


92  PRINCIPLES  OF  THE  TELEPHONE 

After  the  condensers  have  remained  in  the  tank  for  about  an 
hour  the  air  is  again  admitted.  This  forces  the  paraffine  into 
the  remote  recesses  of  the  condensers.  By  hydraulic  presses  the 
condensers  are  next  subjected  to  heavy  pressure  which  removes 
all  excess  paraffine  and  forces  the  plates  together  as  closely 
as  possible.  This  process  increases  the  capacity  of  the  con- 
denser as  is  evident  from  the  formula  given,  which  shows 
that  the  capacity  increases  as  the  thickness,  t,  of  the  dielectric 
decreases. 

The  partially  completed  condensers  are  now  tested  for  capacity 
and  insulation  resistance  at  a  voltage  of  at  least  double  the  work- 
ing value.  Those  passing  this  test  are  next  placed  in  moisture- 
proof  containers.  The  containers  are  lined  with  pasteboard, 
the  condenser  placed  in  position  and  the  case  is  filled  with 
paraffine.  The  terminals  are  next  placed  in  position  and  the 
cover  is  soldered  on,  after  which  a  further  test  is  made  to  check 
the  capacity  and  insulation.  A  finished  condenser  is  shown  in 
Fig.  71. 

115.  Analogy  for  a  Condenser. — A  better  understanding  of 
the  action  of  a  condenser  may  be  had  by  considering  an  analogy. 
Suppose  we  have  an  air  tank  that  under  1  atmospheric  pressure 
holds  a  certain  definite  quantity  of  air,  say  5  Ib.  We  can  define 
the  capacity  of  the  vessel  in  terms  of  the  number  of  pounds  of 
air  it  holds,  and  call  it  a  5-lb.  tank. 

If  the  pressure  is  doubled,  the  tank  will  hold  10  Ib.  of  air.  Since 
we  have  defined  the  capacity  of  the  tank  in  terms  of  unit  (1 
atmosphere)  pressure,  we  can  not  call  it  a  10-lb.  tank.  A  10-lb. 
tank  under  the  same  conditions  will  hold  20  Ib.  of  air. 

Furthermore,  if  the  tank  be  exhausted,  evidently  no  back 
pressure  will  be  exerted  when  air  is  first  admitted  to  the  tank. 
As  soon  as  some  air  is  admitted  to  the  tank,  back  pressure  begins 
to  manifest  itself,  and  when  the  back  pressure  equals  the  maxi- 
mum applied  pressure,  no  more  air  enters  the  tank.  We  thus 
see  that  the  amount  of  air  entering  per  unit  of  time  depends  upon 
the  back  pressure,  and  this  back  pressure  will  depend  upon  the 
capacity  of  the  tank.  For  instance,  if  we  put  5  Ib.  of  air  in  a 
10-lb.  tank,  the  back  pressure  will  be  one-half  as  great  as  when  5 
Ib.  of  air  are  put  into  a  5-lb.  tank.  We  can  then  say  that  unit 
capacity  of  a  tank  is  such  that  when  1  Ib.  of  air  is  forced  into 
it  the  pressure  will  be  equal  to  1  atmosphere.  Evidently  a  cer- 
tain amount  of  work  will  be  done  in  forcing  the  air  into  the 


COMMON  BATTERY  TELEPHONES  93 

tank,  and  we  could  define  unit  capacity  in  terms  of  the  work 
expended. 

The  capacity  of  electrical  conductors  is  analogous  to  the 
capacity  of  the  air  tank  discussed  above.  The  capacity  of  a 
condenser  or  system  of  conductors  is  usually  defined  in  terms  of 
the  quantity  of  electricity  required  to  raise  the  difference  of 
pressure  between  the  terminals  by  1  volt.  In  accordance  with 
this  definition  the  quantity  of  electricity  that  a  condenser  will 
contain  is  equal  to  the  product  of  the  capacity  and  pressure. 

116.  Action  of  a  Condenser. — If  a  condenser  has  one  of  its 
plates  connected  to  each  side  of  a  battery  circuit,  it  will  become 
charged;  that  is,  a  quantity  of  electricity  will  flow  into  the  con- 
denser due  to  the  battery  pressure,  and  one  plate  will  become 
positively  and  the  other  negatively  charged.  After  a  condenser 
has  been  connected  to  a  direct-current  circuit  for  a  short  time, 
there  will  be  no  flow  of  current  to  or  from  the  condenser,  since 
the  condenser  becomes  fully  charged  almost  instantaneously; 
and  when  charged,  the  difference  in  pressure  between  its  plates 
is^the  same  as  that  of  the  battery  or  other  source  of  charging 
current.  If  the  pressure  in  the  circuit  be  decreased  or  reversed, 
the  charge  will  flow  out  of  the  condenser  and  back  through  the 
circuit. 

When  an  alternating  pressure  is  impressed  upon  a  condenser 
the  action  is  somewhat  different.  As  the  pressure  increases  from 
zero  to  a  maximum  a  current  flows  into  the  condenser,  one  side 
becoming  charged  positively  and  the  other  side  negatively. 
The  current  flows  as  long  as  the  pressure  is  changing,  and  the 
back  pressure  of  the  condenser  is  always  just  equal  to  the  applied 
pressure.  When  the  applied  pressure  begins  to  decrease,  the 
current  begins  to  flow  out  of  the  condenser.  When  the  applied 
pressure  is  reversed  the  current  flows  into  the  condenser  in  the 
opposite  direction.  This  continues  until  the  applied  pressure 
.again  attains  a  maximum  value,  when  the  current  again  is  re- 
versed. These  fluctuations  of  current  continue  so  long  as  the 
applied  pressure  fluctuates  or  changes.  An  alternating  current 
may  thus  flow  in  a  circuit  containing  a  condenser.  The  exact 
value  of  such  a  current  will  depend  upon  the  applied  e.m.f.,  the 
frequency,  the  capacity,  and  the  resistance  of  the  circuit.  The 
algebraic  expression  for  a  current  in  a  circuit  having  capacity  and 
resistance  is 


94  PRINCIPLES  OF  THE  TELEPHONE 

E 


I  = 


'      (27T/C)2 

where 

E  =  applied  e.m.f. 

R  =  resistance  in  ohms 

/  =  frequency  of  the  applied  e.m.f. 

TT  =  3.1416 
and        C  =  capacity  in  farads. 

EXAMPLES 

1.  A  pressure  of  110  volts  at  60  cycles  is  impressed  upon  a  circuit  whose 
resistance  is  5  ohms  and  capacity  %  microfarad,  what  is  the  current? 

Solution 
Given 

E  =  110  volts 
R  =  5  ohms 
/  =  60 

C  =  %  X  10~6  farads 
To  find  I 

110 


7  = 


52  + 


(27r  X  60  X  H  X  10-6)2 
110 


V25  +  (2*  x  20) 


110 


,   A/25+  (7,955)2 

110  110 

"  V (7,955)2  ~~  7,955 

as  25  is  negligible  in  comparison  with  (7,955) 2  =  0.013  amp. 

2.  Suppose  that  in  problem  1  the  frequency  were  increased  to  600,  what 
would  the  current  be  then? 

Solution 

The  solution  is  exactly  the  same  as  the  foregoing,  except  for/ we  substitute 
600.     The  equation  for  current  becomes 

/--      =y$= 

3  X  106  \  2 
ITT  X  600/ 
110 


A/25  +  (795.5)2 

=0.13  amp.,  nearly. 


COMMON  BATTERY  TELEPHONES  95 

This  shows  that  when  the  resistance  is  small,  the  current  increases  or  varies 
directly  as  the  frequency  so  long  as  the  pressure  remains  constant.  Both  the 
voice  currents  and  ringing  currents  are  of  high  enough  frequency  to  give  an 
appreciable  current  through  a  condenser.  The  frequencies  of  voice  currents 
range  between  100  and  2,500  cycles  per  second  in  ordinary  telephonic 
communication. 

117.  Function  of  Condenser  in  Telephone  Circuit. — The  func- 
tions of  a  condenser  in  a  telephone  circuit  are  determined  some- 
what by  the  system  of  connections  employed.     The  physical 
basis  for  its  use  is  the  action  of  a  condenser  with  reference  to 
direct  and  alternating  currents. 

The  subscriber's  apparatus  in  the  common  battery  system 
comprises  a  transmitter,  receiver,  and  ringer.  Direct  current  is 
used  to  operate  the  transmitter,  while  alternating  current  is 
preferable  for  the  operation  of  the  receiver  and  ringer.  As  a 
condenser  will  not  permit  the  passage  of  direct  current,  but  will 
permit  the  flow  of  an  alternating  current,  a  condenser  is  con- 
nected into  that  part  of  the  circuit  through  which  only  alternating 
current  is  to  flow.  The  points  of  connection  will  depend  upon 
the  system  of  connections  used.  There  are  several  different 
connections  used  in  practice  of  which  the  following  are  the  most 
common : 

118.  Receiver  and   Transmitter  in   Series;   Condenser  and 
Ringer  in  Series. — What  is  perhaps  the  simplest  connection  is 


Receiver 

FIG.  72. 

indicated  in  Fig.  72,  the  bell  being  bridged  across  the  line  in 
series  with  the  condenser.  Since  the  condenser  will  allow  the 
ringing  current  to  flow  in  the  circuit,  the  instrument  is  ready  to 
receive  signals  from  central  at  any  time  when  the  receiver  is  on 
the  hook.  As  direct  current  from  the  central  battery  can  not 
flow  through  the  condenser,  there  is  no  battery  current  flowing 
as  long  as  the  talking  circuit  is  open. 


96  PRINCIPLES  OF  THE  TELEPHONE 

When  the  subscriber  desires  to  signal  central,  he  merely  re- 
moves the  receiver  from  the  hook,  which  closes  contact  a,  thus 
completing  the  talking  circuit  and  allowing  battery  current  to 
flow  in  his  circuit.  As  soon  as  the  talking  circuit  is  closed,  the 
battery  current  flowing  lights  a  small  electric  lamp  in  the  central 
office,  thus  attracting  the  attention  of  the  operator. 

When  a  circuit  like  that  shown  in  Fig.  72  is  used,  the  talking 
current  flows  directly  through  the  receiver.  If  the  receiver 
should  happen  to  be  connected  in  the  line  the  wrong  way,  the 
effect  of  its  permanent  magnet  would  be  largely  destroyed,  for 
usually  there  is  enough  battery  current  employed  on  a  line  to 
overcome  entirely  the  permanent  magnets  of  a  receiver,  if  it 
flows  through  the  receiver  in  such  a  way  as  to  oppose  them.  A 
receiver  with  line  battery  flowing  through  it  in  this  manner  will 
have  only  about  half  the  efficiency  that  it  should  have. 

119.  Induction  Coil,  No  Condenser  in  Receiver  Circuit. — 
Fig.  73  shows  a  more  common  arrangement  of  the  circuits  of  a 
C.B.  telephone.  The  receiver  as  shown  is  connected  to  the 


Receiver- 


FIG.  73. 


talking  circuit,  through  the  induction  coil  only.  In  this  case  the 
circuits  are  so  arranged  that  only  the  currents  induced  in  the 
secondary  winding,  due  to  the  variations  in  battery  current, 
flow  through  the  receiver. 

120.  Induction  Coil  and  Condenser  in  Ringer  and  Receiver 
Circuits. — Another  system  of  connections  very  extensively  used 
is  that  shown  in  Fig.  74.  This  diagram  shows  that  when  the 
hook  switch  is  open  no  direct  current  can  flow.  Alternating 
current  can,  however,  be  sent  over  the  line  to  operate  the  ringer. 

When  the  receiver  is  removed  from  the  hook  the  receiver  cir- 
cuit is  connected  in  series  with  the  condenser.  In  such  a  connec- 
tion the  condenser  minimizes  the  inductive  effect  of  the  second- 
ary winding  of  the  induction  coil  and  the  receiver  windings, 


COMMON  BATTERY  TELEPHONES 


97 


increasing  the  efficiency  of  transmission.  A  brief  consideration  of 
the  principles  involved  will  make  clear  how  the  condenser  in- 
creases the  sensitiveness  of  the  receiver.  Let  us  consider  the 
receiving  circuit  closed  and  the  subscriber  listening.  Under 
this  condition  the  direct  line  current  flows  through  the  primary 
of  the  induction  coil  and  transmitter.  The  transmitter  offers  a 
fixed  resistance  to  the  flow  of  current.  The  line  current  fluctuates 
in  volume  according  to  the  sound  waves  causing  the  distant  trans- 
mitter to  vibrate.  With  the  substation  circuit  closed  and  the 
transmitter  at  rest  the  condenser  is  charged  to  a  difference  of 


0 


FIG.  74. 

potential  equal  to  that  across  the  transmitter.  Now  when  a 
pulsating  current  flows  in  the  primary,  an  alternating  current  is 
induced  in  the  secondary.  At  one  instant  it  flows  through  the 
receiver  into  the  transmitter  circuit,  and  as  a  reversal  occurs  it 
flows  into  the  condenser  but  does  not  pass  through;  it  is  retained 
there  only  during  the  interval  required  for  the  current  to  reverse 
in  the  coil  when  the  condenser  discharges  into  the  circuit  through 
the  receiver.  This  oscillating  action  of  the  condenser  increases 
the  sensitiveness  of  the  receiver  in  reproducing  the  vibrations 
of  the  distant  transmitter.  This  is  the  main  reason  for  the  use 
of  a  condenser  in  a  receiver  circuit.  In  this  connection  it  per- 
forms two  functions;  to  prevent  direct  current  from  flowing 


98 


PRINCIPLES  OF  THE  TELEPHONE 


through  the  ringer,  and  to  reinforce  the  action -of  the  induction 
coil  and  thus  increase  the  sensitiveness  of  the  receiver.  This 
system  of  connections  is  standard  with  the  American  Telephone 
and  Telegraph  Co.  and  is  widely  used  both  in  this  country  and  in 
England. 

A  modification  of  the  American  Telephone  and  Telegraph 
Company's  system  of  connections  is  shown  in  Fig.  75.  An 
examination  of  this  diagram  will  show  that  the  transmitter, 
the  receiver,  and  the  primary  of  the  induction  coil  are  in  the 
line  circuit,  while  the  secondary,  the  transmitter,  and  the 
condenser  form  a  local  circuit.  When  the  transmitter  is  sta- 
tionary— that  is,  when  the  subscriber  is  listening — the  pulsating 


I 


^wvw^--j 


FIG.  75. 

current  causes  a  variation  in  the  potential  at  the  terminals  of 
the  condenser.  These  variations  in  pressure  cause  the  con- 
denser to  be  charged  and  discharged,  thus  reinforcing  the 
fluctuations  in  the  receiver.  A  similar  action  takes  place  when 
the  transmitter  is  used. 

121.  Retardation  Coil  in  Place  of  Induction  Coil. — The  self- 
inductance  of  a  coil  prevents  the  current  in  the  coil  from  reaching 
a  maximum  value  at  the  .instant  of  maximum  pressure.  The 
growth,  or  increase  or  decrease,  of  current  through  such  a  coil  is 
retarded  in  time,  and  hence  a  coil  with  large  self-inductance  is 
called  a  retardation  coil.  A  retardation  coil  differs  from  an 
induction  coil  in  that  it  contains  only  one  winding.  The  use  of 
such  a  coil  in  the  subscriber's  circuit  is  shown  in  Fig.  76.  The 
function  of  the  retardation  coil  will  be  readily  understood  from 
the  following: 


COMMON  BATTERY  TELEPHONES 


99 


Assuming  the  receiver  on  the  hook,  the  ringing  circuit  may  be 
traced  from  line  conductor  1  to  branch  3,  ringer  4,  conductor  5, 
switch-hook  points  6  and  7,  around  the  receiver  by  way  of  the 
shunt  16,  thence  through  condenser  15  to  the  other  side  of  the 
line  2.  With  the  receiver  off  the  hook,  as  shown  in  the  diagram, 
it  will  be  observed  that  two  parallel  paths  are  provided,  one  con- 
taining the  condenser  and  receiver  and  the  other  the  retardation 
coil  13  and  the  transmitter.  When  the  subscriber  is  listening,  the 
passage  of  the  high-frequency  voice  currents  is  opposed  by  the 
retardation  coil,  but  they  have  practically  free  passage  through 
the  condenser  and  receiver.  The  direct  current  for  the  trans- 


FIG.  76. 

mitter  on  the  other  hand  can  not  pass  through  the  condenser 
but  passes  quite  freely  through  the  retardation  coil.  Such  a, 
combination  of  retardation  or  impedance  coil  and  condenser  pro- 
vides an  automatic  means  of  separating  the  high-frequency  voice 
currents  from  the  direct  current.  No  direct  current  ever  flows 
through  the  receiver.  The  system  of  connections  shown  in  Fig. 
76  is  that  employed  by  the  Kellogg  Switchboard  and  Supply  Co. 
122.  Wheatstone's  Bridge  Connection. — A  very  interesting 
substation  circuit  is  that  shown  in  Fig.  77.  The  principle  of  the 
retardation  coil  is  again  employed  to  keep  the  direct  current  out 
of  the  receiver.  As  shown,  the  circuit  consists  of  four  coils,  two 
retardation  coils,  and  two  noninductive  resistance  coils.  These 


100 


PRINCIPLES  OF  THE  TELEPHONE 


four  coils  are  connected  so  as  to  form  the  four  arms  of  the  WJieat- 
stone  bridge.  The  two  parallel  paths  from  A  to  B  have  the 
same  resistance,  hence  the  steady  direct  current  entering  at  A 
divides,  one  half  passing  by  way  of  ACB  and  the  other  by  way 
of  ADB.  The  potentials  of  the  points  C  and  D  are  equal,  and 
hence  no  direct  current  flows  through  the  receiver.  When, 
however,  high-frequency  voice  currents  enter  at  A,  their  passage 
is  opposed  much  more  by  the  retardation  coil  between  A  and  D 
than  by  the  noninductive  coil  between  A  and  (7;  hence  they  pass 


FIG.  77. 

to  C  through  the  receiver  to  D  and  then  to  B.  Such  a  combina- 
tion of  noninductive  and  inductive  coils,  when  properly  balanced, 
successfully  keeps  the  steady  direct  current  out  of  the  receiver. 
The  condenser  is  placed  in  the  ringing  circuit  only.  This  system 
of  connections  has  been  largely  used  by  the  Dean  Electric  Co. 
in  connection  with  their  common  battery  telephones. 

123.  C.B.  Wall  Sets. — A  common  form  of  wall  set  is  shown  in 
Fig.  78,  this  particular  one  being  of  Kellogg  manufacture.     The 
wiring  diagram  for  this  instrument  is  shown  in  Fig.  76. 

124.  Hotel  Sets. — The  common  battery  hotel  or  residence  set 
is  designed  to  be  used  in  places  where  it  is  desirable  to  economize 
space,  as  are  the  local  battery  sets  of  the  same  type.     These 


COMMON  BATTERY  TELEPHONES 


101 


cabinets  are  made  either  of  wood  or  pressed  steel;  the  latter  have 
been  in  growing  favor  of  recent  years.  A  Stromberg-Carlson 
steel  hotel  set  is  shown  in  Fig.  79.  An  open  view  of  this  set, 
with  a  part  of  the  cover  cut  away  to  show  the  connections  of  the 


FIG.  78. 


transmitter,  is  given  in  Fig.  80,  while  Fig.  81  shows  a  Western 
Electric  hotel  "  phone." 

125.  Desk  Sets. — The  common  battery  desk  set  consists  of 
the  desk  stand,  and  the  desk  box  containing  the  ringer;  the  latter 


102  PRINCIPLES  OF  THE  TELEPHONE 


FIG.  79. 


FIG.  80. 


COMMON  BATTERY  TELEPHONES 


103 


may  be  of  either  wood  or  steel.  In  Fig.  82  are  shown  the  parts 
of  the  Kellogg  desk  set.  The  desk  stand  itself  contains  all  the 
working  parts  of  this  telephone,  except  the  ringer  and  its  con- 


FIG.  81. 


FIG. 


denser.     A  study  of  the  circuit  diagram  of  this  set  in  Fig.  83 
shows  that  two  condensers  are  employed — one  in  the  base  of  the 
desk  stand,  and  the  other,  as  stated  above,  in  the  ringer  box. 
10 


104 


PRINCIPLES  OF  THE  TELEPHONE 


The  reason  for  the  employment  of  two  condensers  is  that  with 
such  an  arrangement  only  two  conductors  are  needed  between 
the  desk  box  and  desk  stand.  With  the  addition  of  extra  con- 
ductors between  the  desk  box  and  desk  stand,  a  single  condenser 
can  be  made  to  serve  in  this  place  as  readily  as  it  does  in  the 
wall  type  previously  discussed,  as  in  all  other  respects  the  wiring 
and  arrangement  of  parts  are  practically  the  same. 

The  general  practice  of  some  other  companies  is  to  mount  only 
the  transmitter,  hook  switch,  and  receiver  in  the  desk  stand,  all 
other  parts  being  mounted  in  the  desk  box.  There  are  many 
other  forms  of  common  battery  telephones,  including  special 


FIG.  83. 


forms  of  wall  telephones,  adjustable  desk  stands,  hand  telephones, 
etc.,  which  we  will  not  discuss  in  this  course,  although  they  have 
a  considerable  field  of  usefulness. 


QUESTIONS 

1.  In  what  ways  does  a  central  battery  telephone  system  differ  from  a 
local  battery  system? 

2.  Do  you  see  any  advantage  in  a  C.B.  system?     In  an  L.B.  system? 
Explain. 

3.  What  is  a  condenser?     How  does  it  work? 

4.  Why  is  a  condenser  necessary  in  a  C.B.  telephone  system?     Explain 
its  action. 

6.  How  are  telephone  condensers  made? 


COMMON  BATTERY  TELEPHONES  105 

6.  What  is  the  method  of  signalling  the  central  operator  in  a  C.B.  system? 

7.  What  is  the  objection  to  having  the  receiver  directly  in  the  talking 
circuit? 

8.  Show  by  diagrams  how  the  circuits  of  a  C.B.  telephone  are  arranged 
so  that  the  receiver  is  not  directly  connected  to  the  line. 

9.  Diagram  and  explain  the  Wheatstone  bridge  method  of  connections. 

10.  A  pressure  of  100  volts  alternating  at   a  frequency  of  60  cycles  is 
connected  to  a  circuit  having  a  resistance  of  1  ohm  and  a  capacity  of  3  micro- 
farads.    What  is  the  current? 

11.  If  the  resistance  of  a  circuit  is  small  in  comparison  with  the  capacity 
reactance  of  a  circuit,  how  does  the  current  in  the  circuit  vary  with  the 
frequency? 

12.  What  is  a  farad?     What  is  a  microfarad? 

13.  What  is  meant  by  dielectric  constant?     If  we  use  a  dielectric  whose 
constant  is  high,  how  will  the  capacity  of  a  condenser  compare  with  the 
capacity  of  one  of  same  size  but  with  air  as  the  dielectric? 

14.  Explain  fully  the  action  of  the  condenser  when  connected  as  shown  in 
Fig.  74. 

15.  Explain  the  retardation  coil.     If  an  alternating  e.m.f.  be  connected 
to  a  retardation  coil,  will  the  current  reach  its  maximum  value  at  the  same 
time  as  the  applied  e.m.f.? 


CHAPTER  XI 
FAULTS  IN  SUBSTATION  TELEPHONE  APPARATUS 

126.  General. — In  order  that  a  telephone  may  give  efficient 
service  any  trouble  or  difficulty  in  operation  must  be  promptly 
located    and    removed.     Troubles    are    called    faults,    and    the 
process  of  locating  the  trouble  is  called  " trouble  shooting"  or 
fault  finding. 

As  the  telephone  is  an  apparatus  which  makes  use  of  me- 
chanical, electrical,  and  magnetic  principles,  trouble  may  develop 
in  any  one  of  these  classes. 

Mechanical  troubles  are  usually  disclosed  by  the  faulty 
operation,  or  nonoperation  of  the  electrical  devices,  hence  in 
their  localization,  electrical  principles  are  used. 

The  most  common  fault  in  an  electrical  apparatus  is  due  either 
to  short  circuits  or  open  circuits.  In  one  way  or  another  these 
cause  most  of  the  telephone  troubles.  Where  batteries  and  a 
magneto  ringer  are  used,  these  may  fail  by  exhaustion,  that  is, 
the  batteries  may  be  used  up;  and  the  magnets  on  the  generator 
may  be  too  weak  or  reversed. 

If  a  telephone  is  in  good  operating  condition,  the  different  parts 
will  behave  under  test  in  a  certain  positive  manner.  The  first 
step  in  localizing  trouble  is  to  perform  what  are  known  as  O.  K. 
or  correct  tests.  These  are  five  in  number,  and  are  as  follows: 

127.  O.  K.  or  Correct  Tests,  Local  Battery  Telephones,  Line 
Disconnected. — 

1.  When  the  magneto  is  turned,  the  bells  should  ring. 

2.  No  generator  current  should  flow  through  the  coils  of  the 
receiver  when  the  hook  is  down. 

3.  A  spark  should  be  seen  at  the  hook-switch  contact  when  it  is 
moved  up  and  down. 

4.  Under  normal  conditions  no  battery  current  should  flow 
through  the  receiver. 

To  test  for  battery  current  in  the  receiver  hold  the  receiver  to 
the  ear  and  short-circuit  the  line  terminals.  If  you  hear  clicks, 
the  battery  current  is  flowing  through  the  receiver. 

106 


SUBSTATION  TELEPHONE  APPARATUS 


107 


L, 


5.  If  in  Fig.  84  points  LI  and  L2  are  bridged  or  short-circuited, 
a  very  strong  side  tone  should  be  heard  in  the  receiver  when  one 
blows  or  speaks  into  the  transmitter. 

128.  Side  Tone. — An  examination  of  Fig.  84  will  make  clear 
what  is  meant  by  side  tone.     If  LI  and  L2  are  short-circuited  and 
the  receiver  raised  from  the  hook,  it  is  evident  that  the  currents 
induced  in  the  secondary  of  the  induction  coil  must  pass  through 
the  receiver;  that  is,  any  sound  causing  a  vibration  in  the  trans- 
mitter can  be  heard  at  the  receiver. 

This  is  called  side  tone.  When  this 
circuit  has  low  resistance,  as  when 
the  line  terminals  are  short-circuited, 
the  sound  given  out  by  the  receiver 
will  be  comparatively  loud,  or  a  strong 
side  tone  will  be  heard. 

129.  Classification    of    Faults. — If 
the   above-mentioned   conditions  are 
not  fulfilled,  there  is  a  fault  in  the 
apparatus    which    must     be    found. 
Trouble   manifests   itself  by  the  in- 
activity or  faulty  operation  of  some 
part  of  the  telephone  set.     This  in- 
activity is  the  symptom  of  trouble, 
and  it  is  more  convenient  to  classify 
the  faults  with  reference  to  the  symp- 
toms  disclosed;  hence   the  following 
classification. 

Bell  Does  Not  Ring. — In  the  series 
type   of   instrument  the ,  subscriber's 

bell  should  ring  whenever  the  generator  crank  is  turned.  If 
the  bell  does  not  ring  under  such  conditions,  it  may  be  due  to 
an  open  line.  To  test  for  open  line,  connect  the  two  line  ter- 
minals of  the  telephone;  if  the  bell  rings  when  the  generator 
is  operated,  the  line  is  open.  If  the  bell  does  not  ring  when 
the  line  terminals  are  short-circuited,  the  fault  may  be  in  the 
generator,  ringer,  switch  contacts,  or  inside  wiring.  Examine 
inside  wiring  and  be  sure  that  all  connections  are  firm  and  that 
there  are  no  broken  wires.  Examine  the  generator  switch  and 
see  that  the  switch  contact  is  open  when  the  crank  is  turned; 
otherwise  the  generator  is  short-circuited  through  this  switch. 
Examine  the  hook-switch  contacts  and  see  that  the  contacts  are 


FIG.  84. 


108  PRINCIPLES  OF  THE  TELEPHONE 

all  clean,  and  that  the  upper  ones  are  closed  when  the  hook  is 
up.  If  no  faulty  contacts  or  connections  are  found,  the  trouble 
is  probably  an  open  circuit  in  the  generator  or  ringer  coil.  Place 
one  finger  on  the  frame  of  the  generator  and  another  on  the 
spring  at  the  end  of  the  armature,  turn  the  crank,  and  see  if  a 
shock  can  be  felt.  If  no  shock  can  be  felt,  an  open  circuit  in  the 
armature  coil  exists.  If  the  generator  proves  to  be  all  right, 
the  ringer  coils  must  be  defective. 

If  a  bridging  instrument  is  being  tested,  failure  to  ring  might 
be  due  to  a  short-circuited  line.  To  test  for  this,  disconnect 
the  line  wires,  and  if  the  bell  rings  the  trouble  is  on  the  line. 
Some  bridging  sets  are  so  arranged  that  when  the  generator  is 
cut  in,  the  ringer  is  automatically  cut  out,  in  order  that  no 
ringing  current  may  be  shunted  from  the  line  by  the  local  bell. 
Be  sure  that  the  instrument  is  not  of  such  a  type  before  making 
further  tests.  If  such  be  the  operation  of  the  set,  of  course  the 
bell  will  not  ring  when  the  crank  is  turned,  and  in  order  to  con- 
tinue the  test  the  ringer  must  be  connected  across  the  line. 
If  disconnecting  the  line  wire  does  not  locate  the  trouble,  pro- 
ceed with  the  test  as  for  the  series  instrument,  examining  the 
switches,  wiring,  etc.,  remembering  that  in  the  case  of  a  bridging 
set  the  generator  switch  must  be  closed  instead  of  opened  when 
the  crank  is  turned. 

If  the  bell  does  not  give  a  strong,  clear  ring,  the  trouble  is 
perhaps  mechanical  rather  than  electrical,  and  the  ringer  should 
be  adjusted  as  directed  under  ringer  adjustments  below. 

Can  Not  Call  the  Central  Operator. — This  condition  may  be  due 
to  weak  or  defective  generator.  To  test,  disconnect  the  line 
wires,  place  the  fingers  across  the  line  terminals  of  the  telephone, 
and  feel  for  current  when  the  crank  is  turned.  If  no  current  be 
felt,  test  the  generator  directly  with  the  fingers  by  placing  one 
on  the  frame  and  another  on  the  spring  at  the  end  of  the 
armature,  as  above.  If  no  current  be  felt,  the  armature  winding 
is  open.  If  current  be  felt,  the  circuit  is  open  in  the  switch  or 
wiring.  If  the  generator  turns  hard,  there  is  a  short-circuit  in 
the  generator  or  some  other  part  of  the  telephone. 

Can  Not  Hear  nor  be  Heard. — This  condition  is  probably  due 
to  an  open  listening  circuit.  Examine  hook  contacts  and  see 
that  the  contacts  are  closed  when  the  hook  is  up.  With  the 
hook  up,  turn  the  generator  crank.  If  the  generator  can  not  be 
heard  in  the  receiver,  the  circuit  is  open.  Short-circuit  the 


SUBSTATION  TELEPHONE  APPARATUS        109 

secondary  of  the  induction  coil  and  ring  as  before.  If  the  gen- 
erator is  now  heard,  it  proves  that  the  secondary  or  the  induction 
coil  is  open.  To  test  the  receiver  and  cord,  place  the  fingers 
across  the  receiver  terminals.  If  the  receiver  be  open,  current 
can  be  felt  when  the  crank  is  turned. 

Can  Hear  but  Can  Not  be  Heard. — This  condition  is  evidently 
due  to  some  fault  in  the  transmitter  circuit.  The  defect  may  be 
due  to  weak  or  worn-out  cells,  poor  contact  at  the  hook  switch, 
an  open  or  short  circuit  in  some  other  part  of  the  primary  circuit 
or  in  the  secondary  of  the  induction  coil. 

First  examine  and  if  possible  test  the  cells  with  an  ammeter. 
If  they  are  found  to  be  in  good  condition,  examine  all  connections 
and  hook-switch  contacts.  To  test  the  transmitter,  short-circuit 
the  line  at  LI  and  L2,  Fig.  84,  and  move  the  hook  up  and  down. 
If  a  spark  appears  at  the  hook-switch  contact  a  when  contact 
is  broken,  test  for  side  tone.  If  the  side  tone  is  found  to  be 
medium,  the  trouble  is  a  weak  transmitter.  If  there  is  no  side 
tone,  or  if  it  is  very  weak,  either  the  transmitter  is  short-circuited 
or  the  primary  of  the  induction  coil  is  short-circuited. 

If,  when  the  hook  is  moved  up  or  down,  no  spark  appears  at 
the  hook-switch  contacts,  then  short-circuit  the  transmitter.  If 
this  gives  a  strong  click  in  the  receiver,  the  transmitter  is  open; 
if  no  click  is  heard  in  the  receiver,  then  short-circuit  the  primary 
of  the  induction  coil  and  move  the  hook  switch  up  and  down. 
If  a  spark  appears,  the  primary  of  the  induction  coil  is  open;  if 
no  spark  appears,  the  wiring  is  open. 

Can  be  Heard  but  Can  Not  Hear. — In  such  cases  the  trouble  is 
usually  in  the  receiver  circuit,  and  is  probably  due  to  a  defective 
receiver  or  to  a  short-circuit  in  the  receiver  cords. 

Intermittent  Faults. — Whenever  there  is  a  complete  break  or 
short  circuit,  the  fault  is  complete  and  lasting  unless  repaired. 
There  is  another  class  of  faults  which  are  more  difficult  to  localize, 
namely,  occasional  faults,  or  those  that  last  for  only  a  short  time, 
while  in  the  interval  the  apparatus  works  satisfactorily.  Such 
faults  are  due,  as  a  rule,  to  loose  contacts  or  open  circuits  which 
may  become  closed  under  vibration,  change  in  temperature, 
movement  of  some  part,  etc. 

In  locating  faults  of  this  nature  careful  inquiries  must  be 
made  concerning  the  circumstances  and  conditions  under  which 
the  fault  appears  or  is  manifest,  and  how  it  affects  the  operation 
of  the  telephone.  One  of  the  most  common  sources  of  inter- 


110  PRINCIPLES  OF  THE  TELEPHONE 

mittent  trouble  is  the  local  cord  circuit.  The  conductors  may 
become  broken,  and  in  a  certain  position  maintain  close  enough 
contact  to  make  transmission  possible,  while  in  other  positions 
the  conductors  may  be  separated  so  as  to  form  a  complete  break. 
Then,  again,  some  of  the  strands  in  one  conductor  may  become 
broken,  pierce  the  covering,  and  form  a  short  circuit  with  the 
other  conductor  while  externally  the  cord  may  show  no  defect 
whatever. 

Test  for  Faulty  Cord. — Whether  or  not  the  cord  is  at  fault  may 
usually  be  determined  by  putting  the  receiver  to  the  ear  and  then 
continuously  blowing  in  the  transmitter,  while  the  cord  is 
pulled,  twisted,  and  wound  in  different  ways.  If  the  fault  is  in 
the  cord,  this  movement  will  cause  interruptions  in  the  noise 
due  to  blowing  into  the  .  transmitter.  Another  method  is  to 
connect  the  cord  and  receiver  to  a  dry  cell  directly,  and  then  to 
listen  while  the  cord  is  pulled,  bent,  and  twisted.  If  there  are 
any  faults  in  the  cord,  they  will  be  disclosed  by  clicks  or  splutter- 
ing sounds  in  the  receiver. 

To  facilitate  the  work  in  localizing  faults  the  following  tabular 
arrangement  has  been  prepared. 

130.  Fault  Finding,  Local  Battery  Telephones,  Substation 
Apparatus. — Disconnect  the  line  wires  before  beginning  the  tests. 
Begin  with  the  five  O.K.  tests  mentioned  at  the  beginning  of  this 
chapter.  If  possible  test  the  cells  with  an  ammeter. 

I.  Bells  ring  weakly. 

A.  Adjust  ringer  and  turn  magneto. 

1.  If  bells  ring  O.K.  the  bells  were  out  of  adjustment. 

2.  If  bells  still  ring  weakly. 

a.  Hold  hook  down  and  turn  magneto. 

(a)  If  there  is  magneto  current  in  receiver,  hook  is 
crossed  with  spring. 

(6)   If  there  is  no   current  in  the  receiver,   the 
magnets  of  the  magneto  are  weak.     This  may  be 
due  to  a  reversal  of  one  or  more  magnets. 
II.  Bells  do  not  ring. 

A.  Bridge  fingers  across  line  and  turn  magneto. 

1.  If  current  is  felt,  the  ringer  coils  are  open,  or  the 
connecting  wires  are  open,  j 

2.  If  no  current  is  felt. 

a.  Short-circuit    magneto    terminals    and    turn    the 
magneto. 


SUBSTATION  TELEPHONE  APPARATUS         111 

(a)  If  it  turns   hard,   magneto   coils   are   short- 
circuited. 

(6)   If   it    turns    easy,    magneto  coils   are   open- 
circuited. 
777.  Can  hear  but  can  not  be  heard. 

A.  Short-circuit  line  at  LI  and  L2. 

1.  If  there  is  a  spark  at  hook  switch  and 
a.  Medium  side  tone,  then 

(a)  Poor  transmitter,  or 
(6)  Weak  dry  cells, 
b.     Weak  or  no  side  tone 

(a)  Transmitter  short-circuited  or 

(b)  Primary  of  induction  coil  is  short-circuited. 

2.  If  there  is  no  spark  at  hook-switch  contacts,  short- 
circuit  transmitter  and 

a.  If,  when  hook  switch  is  closed,  a  strong  click  is 

heard  in  receiver,  then  transmitter  is  open. 
3.  If  no  click  results,  then  short-circuit  primary  coil, 
and 

a.  If  a  spark  appears  at  hook,  primary  coil  is  open. 

b.  If  no  spark  results,  wiring  is  open. 
IV.  Can  not  hear  nor  be  heard. 

A.  Lift  hook  switch  and  turn  magneto. 

1.  If  no  current  flows  in  receiver,  bridge  fingers  across 
receiver  terminals  and  turn  magneto  again. 

a.  If  you  feel  current  effect,  receiver  or  cord  is  open, 

2.  If  you  feel  no   current,   short-circuit  secondary  of 
induction  coil  and  turn  magneto. 

a.  If  there  is  current  in  the  receiver,  secondary  of 
induction  coil  is  open. 

b.  If  no  current  flows  in  receiver,  wiring  is  open. 
131.  Faults  in  Central  Energy  Substation  Instruments.— The 

foregoing  remarks  apply  mainly  to  faults  commonly  found  in 
connection  with  local  battery  apparatus  and  circuits.  In  many 
respects  they  also  apply  to  common  battery  substation  instru- 
ments. As  the  batteries  in  a  common  battery  system  are  not 
located  at  the  substation,  any  faults  in  connection  with  them  are 
quickly  located  and  remedied.  This  fact  also  makes  the  localiza- 
tion of  faults  in  a  common  battery  telephone  somewhat  easier, 
as  a  steady  source  of  current  is  always  assured.  A  common 
source  of  trouble  with  common  battery  circuits  is  in  connection 
11 


112 


PRINCIPLES  OF  THE  TELEPHONE 


with  the  leading-in  wires  where  they  are  fastened  to  damp  walls. 
The  dampness  will  in  time  cause  deterioration  of  the  insulation 
and  grounds  will  result.  Good  practice  requires  that  where  the 
leading-in  wires  pass  through  a  window  or  door  frame  they  should 
be  protected  by  a  porcelain  tube.  The  wires  should  enter  through 
a  hole  sloping  downward  from  within.  Where  the  walls  are 
damp,  the  leading-in  wires  should  be  run  on  porcelain  knobs  so 
as  to  avoid  all  contact  with  the  walls. 

132.  Circuits  of  C.B.  Subscribers*  Telephones.— As  indicated 
in  Figs.  72  and  85,  there  are  two  common  methods  of  connecting 
the  receiver  and  transmitter  in  a  C.B.  subscriber's  set.  In  Fig. 


FIG.  85. 

72  the  receiver  and  transmitter  are  in  series;  that  is,  they  are 
connected  in  such  a  manner  that  the  transmitter  current  also 
passes  through  the  receiver.  In  this  diagram  no  induction  coil 
is  used.  A  similar  arrangement  may  be  employed  with  an  induc- 
tion coil,  as  indicated  in  Fig.  85.  This  arrangement  in  practice 
is  known  as  side-tone  wiring. 

The  other  method  is  that  shown  in  Fig.  86.  This  shows  the 
transmitter  removed  from  the  receiver  circuit,  and  placed  between 
the  hook  and  primary  of  the  induction  coil.  This  arrangement  is 
known  as  side-tone  reduction  wiring.  The  reason  for  these  two 
designations  will  presently  appear.  When  the  arrangement 
shown  in  Fig.  85  is  employed,  the  variations  of  current  in  the 


SUBSTATION  TELEPHONE  APPARATUS 


113 


primary  of  the  induction  coil  induce  currents  in  the  secondary. 
These  secondary  currents  have  a  high  frequency,  hence  can  flow 
quite  readily  in  the  receiver  circuit.  Then,  again,  as  the  current 
in  the  transmitter  varies,  a  variation  in  potential  or  pressure 
will  result  across  the  terminals  of  the  receiver  circuit.  As  the 
potential  increases,  the  condenser  will  be  charged,  and  as  the 
potential  decreases,  the  condenser  will  discharge.  There  are 
thus  two  sets  of  currents  flowing  in  the  receiver  circuit.  When 
the  arrangement  of  Fig.  86  is  employed,  only  the  induced  cur- 
rents flow  in  the  receiver  circuit.  There  will  thus  be  side  tone 


FIG.  86. 

in  either  case,  but  it  will  be  weaker  when  the  side-tone  reduction 
connection  is  used.  Referring  to  Fig.  85  we  see  that  there  are 
three  circuits  in  the  common  battery  telephone  set : 

1.  The  talking  or  battery  circuit  is  from  LI  through  the  primary 
of  the  induction  coil  to  the  hook  switch  at  a,  and  then  through 
transmitter  and  to  L2. 

2.  The  listening  circuit  is  from  b  to  the  receiver,  to  the  second- 
ary of  the  induction  coil  and  condenser  back  to  b. 

3.  The  ringing  circuit  is  from  LI  through  ringer  and  condenser 
to  L2. 

133.  Locating  Faults  in  C.B.  Telephones. — In  testing  a  sub- 
scriber's set  for  faults  some  difficulties  may  be  experienced  which 
are  not  evident  from  the  simplicity  of  the  connections.  An  ex- 


114  PRINCIPLES  OF  THE  TELEPHONE 

amination  of  Figs.  85  and  86  will  show  that  current  may  flow 
through  the  receiver  from  three  directions:  namely,  from  the 
line  through  the  primary  of  the  induction  coil,  from  the  line 
through  the  ringer,  and  through  the  condenser.  The  trans- 
mitter may  receive  current  from  the  line  through  the  ringer,  or 
through  the  secondary  of  the  induction  coil.  These  different 
possible  sources  of  current  may  cause  difficulty  in  locating 
trouble.  The  first  step  in  locating  trouble  is  to  examine  the 
hook  switch  to  see  if  it  makes  good  contact,  if  there  is  current 
on  the  line,  and  also  if  the  trouble  is  in  the  listening  or  talking 
circuits. 

The  most  common  faults  in  C.B.  subscriber's  apparatus  with 
bridged  ringer  may  be  located  in  the  following  manner : 

Can  Not  Call  Central  Operator. — This  is  due  generally  to  line 
trouble  or,  on  party  lines,  to  another  receiver  off  the  hook.  If 
neither  of  the  above  is  the  cause,  examine  hook  contacts.  Hold 
the  hook  down  and  short-circuit  the  receiver  momentarily.  Clicks 
in  the  receiver  'mean  receiver  spring  b,  Fig.  85,  is  crossed  with  hook. 
Next  move  hook  up  and  down  and  look  carefully  for  sparks. 
If  no  spark  appears,  the  other  spring  a  is  crossed  with  the  hook. 
Next  short-circuit  the  condenser.  If  no  spark  be  seen  when  the 
short-circuiting  wire  is  removed,  it  is  a  sign  that  an  internal 
short  circuit  exists  in  the  condenser.  On  desk  sets  a  damp 
cord  will  also  prevent  the  subscriber's  signalling  the  operator. 

Bell  Does  Not  Ring. — This  may  be  due  to  the  bell  being  out 
of  adjustment,  switch-hook  contacts  crossed,  or  an  open  circuit 
in  the  ringer. 

To  adjust  the  ringer  proceed  as  follows: 

First,  loosen  lock  nut  and  adjust  front  bearing  screw,  so  that 
the  armature  will  move  freely  but  not  be  loose.  After  the  ad- 
justment has  been  made,  hold  the  screw  and  tighten  the  lock 
nut. 

Second,  adjust  the  stroke  of  clapper  ball,  if  ringer  be  adjustable 
in  this  respect.  Move  bell  gongs  outward  as  far  as  possible,  and 
adjust  the  armature  so  that  the  clapper  ball  has  a  stroke  of  about 
Min. 

Third,  adjust  the  gongs.  Move  left  gong  toward  the  clapper 
ball  so  that  when  the  left  end  of  the  armature  is  lifted  and 
quickly  released,  the  ball  will  strike  the  gong  once  only  and  will 
not  remain  in  contact  with  it.  Make  the  same  adjustment  for 
the  right  gong. 


SUBSTATION  TELEPHONE  APPARATUS         115 

If  the  bell  be  a  biased  one,  the  biasing  springs  should  next  be 
adjusted.  The  biasing  spring  should  give  sufficient  tension  to 
produce  a  clear  and  even  ring  on  each  gong,  and  should  be 
tightened  or  loosened  to  give  the  desired  effect. 

If  the  bell  does  not  ring  after  adjustment,  ask  the  operator  for 
a  ring,  hold  the  hook  down,  and  see  if  the  generator  is  heard  in 
the  receiver.  If  such  be  the  case,  the  hook  contacts  are  crossed. 
If  no  sound  be  heard  in  the  receiver,  raise  the  hook  and  see  if 
clicks  are  heard  in  the  receiver  when  the  binding  posts  are  short- 
circuited.  If  such  be  the  case,  the  primary  and  secondary  of 
the  induction  coil  are  crossed.  If  none  of  these. tests  locate 
the  trouble,  it  is  probably  due  to  an  open-circuited  ringer  coil. 

Can  Call  Central  but  Can  Not  be  Heard. — This  means  that 
when  the  receiver  is  raised  from  the  hook  the  circuit  through 
the  primary  of  the  induction  coil  and  transmitter  is  complete 
and  that  current  is  on  the  line.  If  under  these  conditions  the 
subscriber  can  not  be  heard,  the  trouble  must  be  in  the  trans- 
mitter, and  as  the  circuit  is  closed  the  transmitter  must  be 
short-circuited  or  packed. 

Can  be  Heard  but  Can  Not  Hear. — This  condition  indicates  a 
fault  in  the  receiver  circuit.  It  may  be  in  the  secondary  of  the 
induction  coil,  the  receiver,  or  the  wiring.  Short-circuit  the 
secondary  of  the  induction  coil  and  listen  for  a  click  in  the 
receiver.  If  no  click  is  heard  in  the  receiver,  either  the  re- 
ceiver or  connections  are  open;  if  a  click  is  heard,  the  secondary 
of  the  induction  coil  is  open. 

These  simple  methods  apply  mainly  to  a  C.B.  telephone  set 
whose  connections  are  shown  in  Figs.  85  and  86.  If  the  con- 
nections differ  radically,  the  general  principles  may  still  apply, 
although  the  procedure  for  localizing  the  fault  may  differ 
somewhat. 

QUESTIONS 

Explain  how  you  would  locate  the  causes  of  the  following  local  battery 
faults : 

1.  Series  bell  does  not  ring. 

2.  Bridging  bell  does  not  ring. 

3.  Can  not  call  operator. 

4.  Can  not  hear  or  talk. 

5.  Can  hear  but  can  not  be  heard. 

6.  Can  talk  but  can  not  hear. 

Explain  how  you  would  locate  the  causes  of  the  following  central  battery 
faults: 

12 


116  PRINCIPLES  OF  THE  TELEPHONE 

7.  Can  not  call  operator. 

8.  Bell  does  not  ring. 

9.  Can  get  central  but  can  not  talk. 

10.  Can  talk  but  can  not  hear. 
Give  complete  adjustments  for: 

11.  Ordinary  polarized  bell. 

12.  Biased  bell. 


CHAPTER  XII 
PROTECTION  OF  TELEPHONE  LINES  AND  APPARATUS 

134.  Need   for   Protection. — Whenever    a    telephone    circuit 
receives  a  voltage  higher  than  that  for  which  it  is  designed, 
excessive  currents  may  flow,  overheating  and  possibly  destroying 
the  apparatus  connected  to  the  circuit.     The  excessive  currents 
also  increase  the  fire  hazard  and  may  cause  injuries  to  persons 
coming  into  contact  with  the  circuit  even  at  some  distance  from 
the  point  of  application  of  the  excessive  voltage. 

135.  Sources  of  Excessive  Voltage. — The  sources  of  excessive 
voltage  against  which  telephone  apparatus  must  be  protected 
may  be  classified  under  the  following  heads : 

1.  Lightning. 

2.  High- volt  age  power  circuits. 

3.  Low-voltage  power  circuits. 

The  low-voltage  currents  may  cause  damage  in  two  ways : 

1.  By  heating. 

2.  By  electrolytic  action. 

To  prevent,  or  at  least  reduce  to  a  minimum,  the  danger  from 
these  sources,  protection  devices  of  various  kinds  are  used. 

136.  Heating  Effect  of  Current— The  heating  effect  of  an 
electric  current  is  proportional  to  the  square  of  the  current 
flowing  and  to  the  resistance  of  the  circuit  within  which  it 
flows.     If  we  wish  to  calculate  the  heat  developed  by  a  given 
current  within  a  given  resistance  in  a  given  time,  we  use  the 
formula 

Heat  =  0.24PRI  calories 

/  is  the  current  in  amperes. 

R  is  the  resistance  of  the  circuit  in  ohms. 

t  is  the  time  in  seconds  during  which  the  current  has  been  flowing. 

A  calorie  is  the  amount  of  heat  required  to  raise  the  temperature 

of  1  gram  of  water  1°C. 

EXAMPLES 

1.  A  current  of  30  amp.  flows  through  a  resistance  of  5  ohms  for  J^  hr. 
How  many  calories  of  heat  are  developed? 
13  117 


118  PRINCIPLES  OF  THE  TELEPHONE 

Solution 

Formula  H  =  0.24/27ft  calories 

7  =  30  amp.,  R  =  5  ohms,  t  =  1,800  sec. 

Then 

H  =  0.24  X  302  X  5  X  1,800 

=  0.24  X  900  X  5  X  1,800 

=  1,944,000  calories. 

2.  Assuming  that  all  of  the  heat  developed  is  utilized  in  heating  water, 
to  what  temperature  would  the  heat  developed  in  the  circuit  mentioned 
in  example  1  raise  10  gal.  of  water? 

Solution 

I  gal.  of  water  weighs  8.33  Ib. 

10  gal.  of  water  weigh  83.3  Ib. 

1  Ib.  =  453.6  grams 

83.3  Ib.  =  83.3  X  453.6  =  about  37,785  grams. 

As  1  calorie  will  raise  the  temperature  of  1  gram  of  water  1°C.,  the  tem- 
perature to  which  1,944,000  calories  will  raise  37,785  grams  is  1,944,000  -f- 
37,785  or  51.4°C.  This  is  the  equivalent  of  92.5°F. 

These  problems  seem  to  indicate  that  the  current  alone  is  re- 
sponsible for  the  heating.  This  view  is  correct,  but  it  must  not 
be  forgotten  that  in  a  given  resistance  the  current  is  directly 
proportional  to  the  pressure  and,  hence,  it  is  just  as  correct  to 
consider  the  heating  effect  to  be  proportional  to  the  square  of 
the  pressure;  for  by  Ohms'  law 

1-^ 

R 

Then 

*-£ 

which,  when  substituted  in  the  formula  for  heat,  gives: 

E2  E2 

Heat  =  0.24  ^  X  Rt  =  0.24  ^  t. 

The  danger  from  excessive  voltage  or  pressure  is  thus  of  two 
kinds :  It  may  cause  excessive  currents,  and  it  may  puncture  the 
insulation,  causing  short  circuits.  Two  kinds  of  protective 
devices  are  thus  necessary,  one  to  prevent  excessive  currents  and 
the  other  to  prevent  the  entrance  of  high  pressures. 

137.  Lightning  Phenomena. — Although  lightning  is  the  oldest 
manifestation  of  the  dissipation  of  large  quantities  of  electrical 
energy,  it  is  the  least  understood  today.  We  shall  not  go  into  the 


TELEPHONE  LINES  AND  APPARATUS          119 

theories  of  lightning,  but  it  may  be  of  interest  to  point  out  certain 
characteristics  of  lightning  and  their  modern  explanation.  The 
old  idea,  one  still  commonly  held,  is  that  lightning  is  a  simple 
discharge  of  electricity  between  clouds,  or  between  clouds  and 
the  earth.  This  conception  is  in  some  respects  inadequate,  al- 
though it  does  seem  to  state  what  one  actually  sees.  Perhaps 
the  more  correct  explanation  of  lightning  phenomena  is  that 
due  to  Dr.  Steinmetz.  His  explanation  is  that  instead  of  being  a 
rupture  of  the  air  under  an  excessive  high  voltage,  lightning  is 
in  reality  an  equalization  of  stresses  within  the  ether.  Lightning 
may  be  compared  to  the  breaking  of  a  piece  of  glass  which  has 
been  rapidly  chilled  and  thereby  filled  with  internal  tension  and 
compression  strains.  If  such  a  piece  of  glass  is  scratched  it 
will  suddenly  break  all  over.  So,  with  our  present  knowledge, 
we  must  consider  as  the  most  probable  explanation — although 
not  certain  by  any  means — that  lightning  discharge  is  the 
phenomena  of  the  equalization  of  internal  electric  stresses  in  the 
cloud,  and  is  analogous  to  the  splintering  or  breaking  of  an 
unevenly  stressed  brittle  material  like  glass. 

If  such  a  discharge,  or  even  a  small  portion  of  it,  happens  to 
pass  through  a  telephone  instrument,  those  parts  connected  with 
the  line  at  that  time  will  probably  be  destroyed  either  by  having 
their  windings  fused  by  the  heavy  current,  or  by  having  the 
insulation  of  the  windings  punctured  by  the  high  voltage.  Of 
course,  the  person  using  the  telephone  when  such  a  stroke  occurs 
would  be  very  fortunate  to  escape  without  serious  injury.  There 
are  two  somewhat  different  lightning  effects  encountered  in 
telephone  work.  The  first  of  these  is  the  direct  stroke,  which 
has  been  discussed  above;  the  second  is  that  due  to  induction. 
Whenever  a  discharge  takes  place,  electromagnetic  waves  move 
out  in  the  ether  somewhat  like  water  waves  in  a  lake  when  a 
disturbance  takes  place  at  some  point.  These  electromagnetic 
waves  when  they  cross  telephone  lines  induce,  currents  in  the 
wires.  The  effect  of  electromagnetic  induction  due  to  a  lightning 
discharge  is  usually  neglected.  There  is,  however,  another  kind 
of  induction  which  must  be  considered,  namely,  electrostatic 
induction.  To  understand  this,  suppose  a  heavily  charged  cloud 
moves  up  to  the  region  over  the  line.  If  the  cloud  is  negatively 
charged,  a  positive  charge  will  be  induced  on  the  telephone  line, 
and  an  equal  negative  charge  will  have  a  tendency  to  pass  to 
earth.  If  the  approach  of  the  cloud  is  slow  enough,  this  free 


120  PRINCIPLES  OF  THE  TELEPHONE 

negative  charge  will  pass  to  earth  by  gradual  leakage  over  the 
insulators.  If  the  approach  of  the  cloud  is  rapid,  and  if  the 
potential  difference  between  its  charge  and  the  earth  is  great, 
the  free  charge  on  the  line  may  puncture  the  insulation  and 
pass  to  earth.  When  the  telephone  line  is  metallic  and  well 
insulated  upon  poles  or  other  fixtures  and  not  connected  with 
the  earth  by  conducting  material,  the  inductive  effect  of  the 
lightning  discharge  will  not  be  so  great  as  upon  a  line  whose 
ends  are  grounded. 

The  telephone  line,  as  a  rule,  does  not  offer  an  easy  and  direct 
path  for  the  lightning  discharge  between  the  cloud  and  the  earth, 
owing  to  its  horizontal  position,  and  usually  receives  only  a  por- 
tion of  the  discharge  which  finds  its  way  to  or  from  the  earth 
over  several  poles  nearest  the  main  path  of  the  discharge.  The 
total  quantity  of  electricity  in  a  lightning  discharge  is  not 
great,  but  as  the  voltage  is  high  the  energy  is  comparatively 
great,  and  as  the  duration  of  the  discharge  is  short,  the  power  is 
very  high. 

138.  Lightning  Conductors. — Before  the  laws  governing  the 
dissipation  of  electrical  energy  were  well  understood,  it  was 
supposed  that  lightning  would  obey  Ohm's  law,  and  that  when- 
ever a  discharge  took  place  it  would  follow  the  easiest  path  to 
earth,  and  that  if  an  easy  path  were  provided  it  would  protect  all 
others.  It  is  not  merely  a  charge  of  electricity  that  must  be 
conducted  to  earth,  but  a  large  quantity  of  energy  must  be 
dissipated  as  quickly  as  possible  and  in  such  a  way  as  not  to  be 
destructive. 

Lightning  is  an  oscillatory  discharge;  that  is,  in  effect  it  can 
be  compared  to  the  action  of  a  compressed  or  extended  spring 
which  is  suddenly  released.  The  spring  does  not  dissipate  its 
energy  in  one  swing  from  the  extended  position  to  its  position  of 
rest,  but  it  overshoots  the  neutral  position  and  oscillates  back 
and  forth  for  some  time.  If,  however,  the  spring  is  immersed 
in  some  viscous  material,  it  will  not  overshoot  its  position  of 
equilibrium,  but  will  dissipate  its  energy  in  moving  slowly 
from  the  extended  position  to  its  position  of  rest.  The  material 
will  not  be  violently  disturbed  and  no  harm  will  be  done. 

In  a  lightning  discharge  there  is  a  certain  amount  of  energy 
to  be  dissipated,  and  it  may  be  that  a  single  rush  of  electricity 
in  one  direction  does  not  suffice  to  dissipate  all  of  the  energy. 
If  the  path  has  a  moderately  high  resistance,  a  single  rush  may  be 


TELEPHONE  LINES  AND  APPARATUS          121 

sufficient,  but  if  the  resistance  of  the  path  is  low  a  single  rush  is 
not  sufficient,  and  the  discharge  will  oscillate  until  all  the  energy 
is  turned  into  heat.  The  rush  in  either  case,  however,  is  likely 
to  be  violent  and  the  discharge  will  not  always  take  the  easiest 
path  but  will  make  its  own  paths,  which  are  sometimes  quite 
unexpected. 

The  relatively  high  resistance  of  iron  as  compared  with  that  of 
copper  makes  the  use  of  iron  wire  for  lightning  rods  on  telephone 
poles  beneficial  in  damping  the  oscillations  of  the  flash,  and  thus 
permitting  the  discharge  to  leak  away  slowly  and  without 
side  flashes.  Its  high  melting  point  and  cheapness  are  also  ad- 
vantages. The  wire  must  not  be  too  small,  however,  or  there 
will  be  risk  of  its  fusing. 

An  electric  current,  like  matter,  seems  to  possess  inertia.  That 
is,  it  takes  some  time  to  start  a  current,  and  likewise  when  its 
flow  has  once  been  established  some  time  is  required  to  reduce  it 
to  zero.  Thus,  when  a  source  of  e.m.f.  is  connected  to  a  circuit, 
the  resulting  current  will  not  at  once,  or  immediately,  reach  a 
maximum  value.  This  is  due  to  the  fact  that  the  establishment 
of  a  current  in  a  circuit  is  accompanied  by  the  storage  of  energy 
in  the  space  surrounding  the  circuit.  To  store  energy  in  the  mag- 
netic field  requires  time.  When  the  circuit  is  broken  the  energy 
is  returned  to  the  circuit  and  thus  does  not  permit  the  current 
to  drop  instantly  to  zero.  This  property  of  a  circuit  is  called 
inductance.  Whenever  a  wire  is  bent,  its  inductance  is  in- 
creased, and  a  greater  opposition  is  presented  to  the  establish- 
ment of  a  current.  It  is  thus  evident  that  wire  used  for  lightning 
rods  should  have  no  sharp  bends,  and  in  fact,  should  be  as  free 
from  bends  as  possible.  As  already  stated,  a  lightning  discharge 
is  of  an  oscillatory  character  with  an  exceedingly  high  frequency. 
This  high  frequency  increases  the  reactance  of  a  bend  or  kink 
to  such  an  extent  that  the  discharge  is  liable  to  jump  across  to 
some  other  conductor  and  not  pass  around  the  bend 

139.  Lightning  Arresters. — There  are  two  ways,  then,  in  which 
lightning  can  affect  a  telephone  circuit :  by  electrostatic  induction 
and  by  direct  stroke.  The  object  of  lightning  arresters  is  to 
protect  the  subscribers'  station  apparatus,  cables,  and  central 
office  equipment  against  damage  from  both  these  causes.  The 
operation  of  lightning  arresters  depends  upon  the  fact  that 
current  due  to  a  lightning  discharge  will  jump  across  a  short  air 
gap  more  readily  than  it  will  pass  through  a  coil  or  other  piece  of 


122 


PRINCIPLES  OF  THE  TELEPHONE 


FIG.  87. 


apparatus  having  considerable  impedance.  It  has  been  stated 
above  that  when  high-frequency  current  flows  in  a  coil  having 
impedance,  a  high  counter-pressure  is  set  up,  which  tends  to 
hold  back  the  current.  As  lightning  has  a  frequency  many  times 
higher  than  that  of  currents  used  in  telephone  practice,  lightning 
will  not  pass  through  the  coils  and  windings  of  telephone  in- 
struments readily  if  some  other  con- 
venient path  to  ground  be  provided. 
In  Fig.  87  is  shown  a  diagram  of  a 
lightning  arrester  and  connections. 
The  two  short  plates  are  connected  to 
the  line;  and  the  long  plate  is  connected 
to  ground.  The  gaps  between  the 
plates  are  made  very  small  so  that  any 

current  due  to  lightning  finds  an  easier  path  to  ground  through 
the  air  gap  than  through  the  coils  of  the  instrument.  Arresters 
were  formerly  made  with  metal  blocks  of  the  general  form  shown, 
but  proved  to  be  quite  unsatisfactory  on  account  of  the  fact 
that  heavy  discharges  were  likely  to  fuse  the  plates  and  fill 
the  gaps  with  molten  metal, 
thus  destroying  the  arrester 
as  well  as  putting  the  line 
out  of  commission. 

140.  Carbon  Block  Ar- 
resters.— The  carbon  block 
arrester  is  in  common  use  on 
account  of  the  fact  that  dis- 
charges between  the  carbon  CARB 
blocks  do  not  melt  or  fuse 
the  blocks  readily;  hence 
the  arrester  is  not  difficult  to 
maintain.  One  form  of  car- 
bon arrester  is  shown  in  Fig. 
88.  This  arrester  consists 


CARBON 


GROUND 

PLATE: 


FIG.  88. 


of  carbon  blocks,  having  the  two  inside  ones  connected  to  the 
ground,  and  the  outside  blocks  connected  one  to  each  side  of  the 
line.  Between  the  outside  and  inside  blocks  are  placed  separa- 
tors of  mica,  which  are  perforated  with  a  number  of  circular  holes 
through  which  the  discharge  takes  place  when  the  arrester  oper- 
ates. Forms  of  micas  are  shown  in  Fig.  89. 

The  American  Telephone  and  Telegraph  Co.  uses  two  types  of 


TELEPHONE  LINES  AND  APPARATUS 


123 


open  space  "cut-outs,"  as  the  carbon  block  arresters  are  some- 
times called. 

The  cut-out  employed  at  substations  and  at  the  central  office 
consists  of  small  carbon  blocks,  one  of  which  is  connected  to  the 
telephone  circuit  and  the  other  to  earth,  separated  by  thin  sheets 
of  mica  0.0055  in.  thick.  A  small  cavity  in  one  of  the  opposite 
faces  of  the  carbon  is  filled  with  a  button  of  fusible  metal  which 
melts  at  about  160°F.  A  carbon  with 
the  fusible  button  is  shown  in  Fig.  90. 

"  The  distance  between  these  carbons  is 
such  that  electricity  at  over  350  volts  will 
pass  from  the  carbon  connected  with  the 
telephone  circuit  across  the  space  to  the 
opposite  carbon  and  thence  to  earth. 
When  this  escape  of  current  to  earth 
takes  place  a  tiny  arc  in  the  space  be- 
tween the  carbons  may  be  sufficient  to  warm  the  carbons  and 
cause  the  fusible  metal  to  flow  from  its  recess  and  fill  the  space 
between  the  carbons  and  thus  establish  a  permanent  connection 
to  ground.  If  the  escape  of  current  to  earth  does  not  sufficiently 
heat  the  carbon  to  cause  the  fusible  metal  globule  to  flow,  a 
permanent  connection  to  ground  may  not  be  established.  In 


FIG.  89. 


FIG.  90. 


many  instances,  however,  even  if  the  fusible  metal  globule  does 
not  fuse,  small  particles  of  carbon  from  the  carbon  blocks  are 
broken  loose  by  the  sudden  current  discharge  and  these  may 
partially  ground  the  line. 

"For  the  protection  of  aerial  and  underground  cable  conductors 
extended  by  open  wires  over  J^  mile  in  length,  the  open  space 
cut-out  is  not  as  sensitive  as  the  one  above  described.  In  this 


124 


PRINCIPLES  OF  THE  TELEPHONE 


case  the  discharge  surfaces  consist  of  two  metal  blocks  and  these 
are  separated  by  means  of  a  mica  0.011  in.  in  thickness." 

The  Kellogg  Co.  equips  its  magneto  telephones  with  carbon  ar- 

resters  of  the  type  shown  in  Figs. 

91a  and  916.  The  metal  back 
plates  are  semicircular,  and  are 
connected  to  the  line.  The  front 
disk  of  carbon  is  connected  to 
ground  and  is  separated  from 
the  line  plates  by  a  thin  sheet 
of  perforated  mica. 
FIG.  9l«.  141.  Self-cleaning  Arresters. 

— Continual  electrical  discharges 

between  blocks  are  likely  to  cause  deposits  of  fine  carbon  dust 
which  interfere  with  the  operation  of  the  arrester.     This  can  be 


FIG.  916. 

removed  readily,  however,  by  taking  the  arrester  apart  and 
cleaning  the  parts.  To  obviate  the  necessity  for  frequent  clean- 
ing a  cut-out  has  been  devised  in  which 
the  discharge  gap  is  wedge-shaped,  be- 
ing narrower  at  the  top  than  at  the 
bottom.  It  is  claimed  that  the  carbon 
particles  will  not  lodge  between  the 
carbons  when  such  a  gap  is  used.  The 
construction  of  the  Roberts  "  self-clean- 
ing arrester"  is  shown  in  Fig.  92. 

A   self-cleaning  arrester  for  outdoor 

installation  is  shown  in  Fig.  93.  This  arrester  has  three  carbon 
blocks,  one  of  which  is  connected  to  ground  and  one  to  each  side 
of  the  line. 


FIG.  92. 


TELEPHONE  LINES  AND  APPARATUS 


125 


There  are  numerous  arresters  on  the  market,  but  they  are 
practically  all  of  the  carbon  type,  and  work  on  the  same  principles 
as  those  outlined  above.  Another  make  is  shown  in  Fig.  94. 

142.  Location  of  Lightning  Arresters. — Arresters  may  be  made 
a  part  of  the  instrument,  or  connected  to  the  line  at  the  point  of 
entrance    to    the    building.     Good    practice    requires  'that    an 
arrester  be  placed  in  the  latter  position  if  thft  line  inside  the  build- 
ing is  of  any  considerable  length. 

143.  Protection  against  Power  Circuits. — The  voltage  of  light- 
ning and  power  circuits  is  always  higher  than  that  of  the  tele- 
phone line;  hence  it  is  neces- 
sary   to    protect    the    latter 
against  possible  contact,  either 
partial  or  complete,  with  the 
former.     To   accomplish  this 
several    types    of    protective 
devices  are  used.     These  de- 


FIG.  93. 


FIG.  94. 


vices  must  protect  against  an  almost  infinite  number  of  possible 
conditions  of  voltage  and  current  strength. 

The  protection  against  damage  from  crosses  with  high-pressure 
power  circuits  is  accomplished  by  means  of  the  same  devices  as 
used  for  guarding  against  damage  by  lightning  or  static  discharges, 
namely,  the  open  space  cut-out.  The  open  space  cut-out  operates, 
as  previously  explained,  by  grounding  the  line.  If  the  disturbing 
voltage  is  of  short  duration  such  as  a  lightning  discharge  no 
other  protection  is  necessary.  When,  however,  the  disorder 
is  due  to  a  cross  with  a  high-tension  or  other  power  line,  the 


126 


PRINCIPLES  OF  THE  TELEPHONE 


resulting  current  will  probably  continue  to  flow  even  if  the  tele- 
phone circuit  is  grounded.  In  fact,  the  grounding  may  even 
increase  the  current  by  reducing  the  resistance.  In  order  to 
guard  against  such  accidents  fuses  are  used. 

144.  Fuses. — A  fuse  is  a  piece  of  conductor  made  of  an  alloy 
having  a  low  melting  point  and  forming  part  of  the  circuit. 
The  action  of  fuses  depends  upon  the  heating  effect  of  the 
electric  current  flowing  through  them.  They  are,  therefore, 


FIG.  95. 

used  not  to  protect  against  high  voltages,  but  to  guard  against 
the  flow  of  currents  which  might  damage  an  instrument  by 
overheating  the  windings  of  its  various  parts.  Fuse  wires  are 
designed  to  melt  when  a  current  above  a  predetermined  strength 
flows,  thus  opening  the  circuit  and  breaking  the  current  before 
the  parts  of  the  instrument  are  overheated. 

A  fuse  does  not  offer  much  protection  against  lightning,  as  a 
lightning  discharge  may  destroy  an  instrument  before  the  tern- 


Frank  B.  Cook— SAnips!-- 600.  Volts'. 
FIG.  96. 

perature  of  the  fuse  is  high  enough  to  melt  it,  or  it  may  even 
jump  across  the  gap  made  by  a  blown  fuse.  Fuses  used  in  tele- 
phone circuits  are  invariably  of  the  enclosed  type.  The  mica 
fuse  having  a  small  copper  wire  between  two  sheets  of  mica,  as 
shown  in  Fig.  95,  is  quite  commonly  used.  The  fuse  wire  is 
attached  to  metal  terminals  at  each  end,  by  means  of  which  the 
fuse  is  held  in  the  block. 

Another  type  of  enclosed  fuse  consists  of  a  fusible  wire  con- 
tained within  a  tube  of  fiber  or  porcelain.     Usually  the  wire  is 


TELEPHONE  LINES  AND  APPARATUS 


127 


surrounded  by  some  nonconducting  powder  to  assist  in  destroy- 
ing the  arc  when  the  fuse  wire  is  vaporized.     An  enclosed  fuse  is 


FIG.  97. 


shown  in  Fig.  96,  and  a  porcelain  fuse   block,  with  fuses,  is 
shown  in  Fig.  97. 

145.  Protectors. — Arresters  and  fuses  are  used  singly  or  in 
combination,  and  are  known  as  protectors. 


FIG.  98. 


The  Western  Electric  58A  protector,  shown  in  Fig.  98,  affords 
protection  against  lightning,  high  voltage,  and  heavy  power 
currents.  It  consists  of  a  carbon  block  arrester,  designed  to 


128  PRINCIPLES  OF  THE  TELEPHONE 

operate  at  400  volts,  and  two  enclosed  fuses  designed  to  carry  5 
amp.  continuously  and  to  operate  at  7  amp.  As  protection 
against  a  continued  arc  after  a  lightning  or  high-voltage  discharge, 
a  plug  of  lead  is  placed  in  the  outside  arrester  blocks.  The 
operation  of  this  arrester  is  as  follows :  Assume  that  one  side  of 
the  line  has  come  into  contact  with  a  600-volt  trolley  wire.  The 
potential  of  this  line  is  immediately  raised  to  that  of  the  trolley 
wire,  and  the  arrester  operates.  At  the  time  of  the  operation 
of  the  arrester,  an  electric  arc  is  set  up  between  the  two  plates. 
This  arc  has  a  very  much  lower  resistance  than  that  of  the  original 
air  gap;  hence  the  current  flow  through  the  arrester  tends  to 
increase.  If  this  rises  to  a  value  of  above  7  amp.,  the  fuse  will 
blow  in  a  short  time,  disconnecting  the  arrester  and  telephone 
instrument  from  the  line.  However,  should  this  current  only 
reach  a  value  of  5  or  6  amp.,  the  fuse  might  not  blow  for  some 


m 


FIG.  99. 

time,  in  which  case  the  fusible  plug  of  the  arrester  would  be 
melted  by  the  heat  of  the  arc,  and  the  metal  would  run  down 
between  the  two  arrester  plates,  short-circuiting  them  and 
allowing  a  large  current  to  flow.  This  large  current  would  at 
once  cause  the  fuse  to  operate  and  disconnect  the  arrester  from 
the  line.  After  the  fusible  plug  has  been  melted  the  arrester 
must  be  taken  apart  and  the  metal  removed  before  the  arrester 
can  be  put  into  service  again. 

Fig.  99  shows  another  type  of  protector  which,  in  addition  to 
the  arrester  and  fuse,  consists  of  a  switch  by  which  the  telephone 
can  be  entirely  disconnected  from  the  line  during  the  time  of  a 
storm.  The  protector  as  shown  is  supposed  to  take  care  of  one 
side  of  the  line  only,  and  is  sufficient  for  grounded  lines.  On 
full  metallic  lines,  however,  a  double-pole  arrester  of  the  same 
type  is  used. 

146.  Protection  against  Weak  Currents. — Very  weak  currents, 
usually  called  "sneak"  currents,  may  flow  if  the  telephone  line 
is  crossed  with  low-voltage  power  or  lighting  lines,  or  with 
comparatively  high-voltage  lines  through  a  high  resistance. 


TELEPHONE  LINES  AND  APPARATUS 


129 


In  central-energy  systems  these  sneak  currents  may  be  caused 
by  a  ground  on  one  of  the  line  wires,  or  the  crossing  of  two  wires 
without  being  subjected  to  a  foreign  potential.  The  danger 
from  such  currents  lies  in  the  fact  that  the  heat  generated  by 
them  accumulates  and  thus  raises  the  temperature  of  the  coil 
through  which  they  flow  to  an  excessively  high  value.  The 
accumulated  heat  causes  deterioration  of  the  insulation  and  may 
cause  open  circuits. 

In  order  to  protect  against  currents  that  are  too  small  to 
operate  the  fuse,  and  which  are  harmful  only  when  permitted  to 
flow  for  a  considerable  time,  a  circuit  grounding  device  called  a 
heat  coil  is  used.  The  heat  coil  consists  of  a  coil  of  fine  German 
silver  wire  which  forms  a  part  of  the  telephone  circuit,  Fig.  100. 

The  general  principles  of  the  operation  of  heat  coils  will  be 
readily  understood  by  reference  to  Fig.  101,  which  shows  a 


FIG.  100. 


FIG.  101. 


section  of  one  type  of  this  protective  device.  The  coil  A  is 
wound  on  a  metal  bobbin  D  which  is  soldered  to  a  metal  stud  Q. 
The  sneak  current,  on  passing  through  the  coil,  heats  the  bobbin, 
and  if  it  flows  for  a  sufficient  length  of  time  the  accumulated 
heat  will  melt  the  solder  when  the  stud  Q  is  pushed  away  by  the 
glass  rod  C  breaking  the  circuit.  The  glass  rod  is  actuated  by 
the  spring  S.  To  ground  the  current  the  lug  Q  is  pushed  against 
a  grounding  contact. 

In  some  makes  of  heat  coils  the  soldered  connection  is  under 
tension.  When  the  solder  melts,  a  spring  breaks  the  circuit 
through  the  apparatus  and  grounds  the  line  by  making  contact 
with  a  grounded  lug.  This  type  of  heat  coil  will  be  more  fully 
explained  when  the  protection  of  central  office  equipment  is 
taken  up. 

The  action  of  the  heat  coil  is  to  protect  the  apparatus  against 
prolonged  currents  of  just  sufficient  strength  to  overheat  its 
windings,  such  currents  being  below  1  amp.  and  consequently 


130  PRINCIPLES  OF  THE  TELEPHONE 

below  the  range  of  practical  fuse  operation.  For  local  battery 
systems  the  heat  coils  are  made  very  sensitive,  since  they  have  to 
protect  apparatus  which,  on  account  of  its  high  resistance  and 
low  heat  conductivity,  may  be  injured  by  comparatively  weak 
currents.  In  this  system  the  telephone  and  ringing  currents, 
which  under  ordinary  conditions  flow  over  the  line  between  a 
substation  and  the  central  office,  will  not  operate  the  heat  coil. 
The  telephone  current  is  of  minute  strength  and  the  ringing  cur- 
rent, though  greater,  is  of  comparatively  short  duration.  The 
quantity  of  heat  developed  by  these  currents  in  the  heat  coil  is 
so  small  that  there  is  no  danger  of  its  operation. 

In  the  common  battery  systems  the  line  from  the  central  office 
to  the  substation  carries,  in  addition  to  the  ringing  and  telephone 
currents,  the  direct  current  for  the  transmitter.  If  the  line  is  of 
very  low  resistance,  this  current  may  attain  a  considerable 
strength.  For  this  reason  it  has  been  necessary  to  provide  a 
heat  coil  of  sufficiently  low  resistance  to  carry  this  current  with- 
out operating,  but  in  addition  all  the  pieces  of  the  telephone 
apparatus  in  the  line  circuit  have  been  designed  so  as  to  withstand 
the  heating  effect  of  this  current  for  an  indefinite  time. 

The  resistance  of  the  sneak  current  arrester  for  local  battery 
systems  is  about  46  ohms.  The  effect  on  telephone  transmission 
of  the  resistance  of  these  arresters  in  the  line  has  been  made  the 
subject  of  carefully  conducted  experiments,  and  it  has  been  found 
that  their  effect  is  quite  imperceptible  on  local  service  transmission. 
On  the  other  hand,  in  order  to  secure  high-efficiency  telephone 
transmission  in  common  battery  systems,  it  is  necessary  that  the 
resistance  be  kept  as  low  as  possible.  The  sneak  current  arrester 
for  this  system  is  designed  with  a  resistance  of  about  3.6  ohms. 
This  is  found  sufficiently  high  to  develop  the  necessary  amount  of 
heat  to  cause  the  arrester  to  operate  promptly  on  dangerous 
currents. 

147.  When  Substations  Need  Protection. — As  regards  the 
necessity  for  protection,  subscribers'  stations  are  of  two  types; 
exposed  and  unexposed.  An  exposed  station  is  one  which  is 
liable  to  be  affected  by  lightning  discharges,  or  by  the  line  coming 
into  contact  with  high-voltage  transmission  and  other  electric 
light  and  power  wires.  Ordinarily,  therefore,  the  line  of  any 
subscriber's  station  which  is  connected  with  the  central  office 
through  aerial  wiring  or  cable  is  considered  exposed.  A  station  is 
likewise  considered  exposed  where  a  building  is  fed  by  a  tap  from 


TELEPHONE  LINES  AND  APPARATUS          131 

an  aerial  cable,  even  though  this  tap  may  be  carried  underground. 
Wherever  a  station  is  so  located  that  electric  light  wires  or  other 
circuits  carrying  a  pressure  of  over  250  volts  are  liable  in  case  of 
failure  to  come  into  contact  with  the  wall  wiring  of  the  telephone, 
the  station  should  be  considered  as  exposed,  and  the  substation 
should  be  protected. 

Subscribers'  stations  connected  directly  to  the  central  office 
through  an  underground  cable  are  considered  unexposed,  and 
therefore  need  no  protection. 


CHAPTER  XIII 


INSTALLATION       . 

148.  Entrance  Holes. — Before  making  any  holes  for  the  en- 
trance of  the  leading-in  wires,  the  location  of  the  protector  must 
be  decided  upon.  Having  decided  upon  the  location  of  the 
protector,  one  entrance  hole  sloping  downward  from  within 
should  be  made,  care  being  taken  that  the  distance  between 
the  protector  and  the  entrance  hole  is  as  short  as  possible.  In 
locating  entrance  holes  and  protectors,  it  is  desirable  to  locate 


LINE. 


NOT  LESS 
THAN  I 


nq   i 
NOTE:- 

THE:  SPACING  OF  PORCELAIN 
SUPPORTS    SHALL  NOT  EJCCE.E.O  18" 
SUPPORTS   SHALL,  BE  PLACED 
APPROXIMATELY    2' FROl^  CORNERS 


JOIST 


\ 

V/o 

Th 

T 
'A 

FIG    2 

LESS 
N    1" 

FIG.  102. 

both  so  as  to  give  the  shortest  and  most  direct  connection  for 
the  ground  wire. 

149.  Leading-in  Wires. — The  wires  extending  from  the  pole 
or  fixture  to  the  building  should  be  attached  to  a  support  on  the 
outside  of  the  latter.  The  distance  from  the  last  outside  support 
to  the  entrance  hole  should  not  be  over  1  ft.,  Fig.  102.  From 

132 


INSTALLATION 


133 


the  last  support  a  twisted  pair  of  wires  should  be  used  to  enter 
the  building.  To  prevent  water  following  the  wire  into  the 
building,  a  drip-loop  should  be  made  at  the  leading-in  wires  at 
a  point  immediately  below  the  entrance  hole,  Fig.  103.  All 
entrance  holes  should  have  porcelain  tubes  for  the  protection  of 
the  leading-in  wires,  and  these  tubes  should  project  a  short 
distance  from  the  hole.  The  leading-in  wires  should  not  come 
into  contact  with  any  part  of  the  building;  and  if  these  wires 
must  be  extended  through  walls,  floors,  or  partitions,  they  should 
be  enclosed  in  porcelain  tubes  in  the  same  manner  as  in  passing 
through  the  outside  wall.  Porcelain  tubes  should  always  be 
firmly  secured  so  that  they  will  not  slip  out  of  place.  If  it  is 
necessary  to  carry  the  leading-in  wires  any  distance  inside  the 


FIG.  103. 

building  before  they  reach  the  protector,  they  should  be  supported 
by  porcelain  knobs  or  cleats. 

150.  Location  of  Protector. — The  protector  should  be  mounted 
upon  the  wall  in  such  a  manner  that  the  fuses  are  vertical,  and 
should  be  placed  as  near  as  possible  to  the  point  where  the  lead- 
ing-in wires  enter.     It  is  essential  that  the  protector  be  not  ex- 
posed to  water  or  dampness;  if  such  is  the  case,  a  protector  de- 
signed for  outside  service  should  be  used.     The  protector  should 
be  mounted  away  from  all  combustible  materials. 

151.  The  Inside  Wiring. — The  wires  used  on  the  inside  of  the 
building  after  the  protector  has  been  passed  should  be  what  is 
known  as  inside  wire,  and  may  be  either  single,  double,  or  triple 
conductor  wire,  depending  upon  the  requirements.     As  neatness 
is  a  desirable  characteristic  of  all  inside  wiring,  it  is  essential  that 
wires  should  be  run  only  horizontally  and  vertically,  and  in 

14 


134  PRINCIPLES  OF  THE  TELEPHONE 

as  workmanlike  manner  as  possible.  As  far  as  possible,  such 
wires  should  be  concealed.  Wherever  picture  molding  is  pro- 
vided, wires  may  be  conveniently  carried  along  this  molding; 
or  if  the  latter  is  not  available,  they  may  be  carried  along  the 
mopboard,  in  corners,  etc.,  but  should  never  cross  open  walls 
or  ceilings. 

All  wire  must  be  fastened  in  such  a  manner  as  not  to  injure 
its  insulation.  For  this  purpose  insulated  staples,  cleats,  or 
insulated  tacks  may  be  used. 

Telephone  wires  should  never  be  run  through  hollow  partitions, 
under  floors,  or  other  places  where  there  is  any  liability  of  coming 
into  contact  with  electric  light  wiring.  When  it  is  necessary  to 
cross  any  open  electric  light  or  power  wire,  pipes,  or  other  con- 
ducting material,  the  telephone  wires  should  not  come  within 
2  in.  of  these  wires  or  pipes,  and  should  be  protected  by 
porcelain  tubes,  or  several  wrappings  of  friction  tape.  Whenever 
practicable,  wires  should  be  run  above  pipes  and  conducting 
materials  which  it  is  necessary  for  them  to  cross.  There  should 
be  no  coils  or  knots  made  in  any  of  the  wires  at  the  protector  or 
telephone  set  terminals,  or  any  other  part  of  the  inside  wiring. 
Where  necessary  to  splice  wires  of  the  system  within  the  build- 
ing, all  joints  should  be  soldered  and  carefully  wrapped  with 
rubber  and  friction  tape.  If  tracer  wires  or  wires  of  different 
colors  are  used,  corresponding  wires  should  always  be  spliced 
together. 

152.  Ground  Wiring. — In  order  to  secure  the  best  service  from 
protectors,  it  is  necessary  that  the  ground  wire  be  properly  in- 
stalled. The  ground  wire  should  run  as  directly  as  possible  from 
the  protector  to  ground,  and  should  have  no  kinks,  coils,  knots, 
or  sharp  bends.  Where  necessary  to  carry  a  ground  wire  through 
an  outside  wall,  a  separate  hole  should  be  provided  at  least  3  in. 
distant  from  the  entrance  hole.  If  necessary  to  protect  this 
wire  from  injury,  it  should  be  protected  by  a  wooden  molding  or 
enclosed  in  a  nonmetallic  conduit,  and  never  run  in  an  iron 
pipe. 

The  ground  connection  may  be  made  through  a  water  or  gas 
pipe,  or  to  a  ground  rod  driven  in  permanently  damp  earth. 
Whenever  connections  are  made  to  pipes,  preference  should  be 
given  to  water  pipes;  and  when  made  to  gas  pipes,  should  be  at 
a  point  between  the  meter  and  the  street  so  that  the  removal  of 
the  meter  will  not  break  the  ground  connection.  When  ground 


INSTALLATION  135 

connections  are  made  to  pipes,  some  form  of  clamps  is  used.  Fig. 
104  shows  a  good  form.  Steam  or  hot-water  pipes  or  other  parts 
of  heating  systems  are  not  desirable  as  ground  connections.  If 
it  be  necessary  to  use  a  ground  rod,  the  latter  should  be  located 
within  the  building  if  possible.  In  connecting  the  ground  wire 
to  the  ground  pipe  or  rod,  the  pipe  or  rod  should  be  thoroughly 
clean,  the  ground  wire  wrapped  around  it  a  number  of  times,  and 
the  connection  soldered. 

153.  Location  of  Telephone  Set. — A  wall  set  should  be  so 
located  that  the  mouthpiece  of  the  transmitter  will  be  at  the 
most  convenient  height  for  the  average  person  using  the  same. 
The  height  from  the  floor  to  the  center  of  the  transmitter  should 
be  about  4  ft.  10  in.  A  telephone  set 
should  not  be  located  where  it  will 
be  injured  by  doors  or  movable  furni- 
ture, or  where  it  will  interfere  with 
persons  passing  through  the  room. 
The  set  should  not  be  mounted  on  a 
damp  wall,  if  the  same  can  be  avoided, 
nor  near  a  window  that  is  liable  to 
be  opened  during  a  storm.  However, 
if  such  be  the  only  space  available,  a  FlG  104 

waterproof  board  should  be  mounted 

on  the  wall,  and  the  telephone  in  turn  mounted  on  this.  Vibrat- 
ing partitions  and  noisy  locations  should  be  avoided. 

The  wall  sets  should  be  fastened  firmly  and  securely  to  the 
wall.  In  attaching  a  set  to  a  wooden  or  plastered  wall,  round- 
head wood  screws  should  be  used.  To  fasten  a  set  to  brick, 
cement,  or  stone  walls,  holes  should  be  drilled  in  the  wall  in 
proper  position  for  the  screws.  The  holes  should  then  be 
plugged,  and  the  set  fastened  with  round-head  wood  screws. 
Care  should  be  taken  that  thoroughly  dry  woocl  is  used,  and  that 
the  plugs  are  large  enough  to  hold  securely.  Expansion  bolts 
may  be  used  in  place  of  the  above.  If  a  set  is  to  be  attached  to 
a  hollow  tile  wall,  holes  should  be  drilled  at  the  proper  places  and 
toggle  bolts  used  to  attach  the  same. 

A  desk  stand  should  be  placed  where  it  is  most  accessible  and 
convenient  for  the  subscriber.  If  possible,  the  bell  box  should 
be  so  located  that  the  cord  can  be  connected  directly  to  the 
terminals  in  the  box,  so  as  to  prevent  the  cord  from  lying  on  the 
floor  or  where  it  might  be  exposed  to  dampness  or  damage. 


136  PRINCIPLES  OF  THE  TELEPHONE 

QUESTIONS 

1.  Why  is  it  necessary  to  protect  telephone  instruments  from  lightning 
and  high-voltage  light  wires? 

2.  What  is  the  principle  of  operation  of  lightning  arresters?     Why  will 
lightning  jump  across  a  small  air  gap  rather  than  pass  through  a  telephone 
instrument? 

3.  Explain  the  construction  of  the  carbon  block  arrester.     What  are 
its  advantages? 

4.  Explain  the  construction  of  fuses  and  give  their  uses. 

6.  Describe  and  explain   the  operation   of  the   Western   Electric   58A 
protector. 

6.  What  is  meant  by  exposed  and  unexposed  subscribers'  stations? 

7.  When  a  telephone  is  being  installed,  where  should  the  protector  be 
located? 

8.  How  are  the  leading-in  wires  carried  through  outside  walls,  partitions, 
etc.? 

9.  How  should  inside  wiring  be  done? 

10.  How  should  the  ground  wire  be  run,  and  the  ground  connections 
made? 

11.  What  precautions  should  be  observed  in  locating  the  telephone  set? 

12.  Why  are  the  leading-in  wires  not  allowed  to  touch  any  part  of  the 
building,  while  the  inside  wiring  may  be  run  along  molding,  etc? 

13.  Explain  the  function  and  action  of  sneak-current  arresters. 

14.  What  is  the  difference  between  sneak  arresters  for  common  battery 
systems  and  for  local  battery  systems? 


CHAPTER  XIV 
PARTY  LINES 

154.  Definition. — The  simplest  form  of  a  telephone  installation 
is  a  line  to  each  end  of  which  is  connected  a  subscriber's  telephone 
set,  Fig.  105.  It  is  evident  that  other  telephones  may  be  bridged 


( 

JT                                                               A 

-~>   i 

r 

oo 

o. 

OO 

o 

FIG.  105. 


across  the  line  anywhere  between  the  two  ends,  or  that  a  branch 
line  may  be  run  from  some  point  on  the  main  line  and  one  or  more 
telephones  connected  to  the  branch  line,  as  shown  in  Fig.  106. 


FIG.  106. 

Telephone  lines  connected  so  that  more  than  one  subscriber  can 
be  called  on  the  same  line  are  known  as  party  lines. 

155.  Classification  of  Party  Lines. — When  telephone  service  is 
supplied  to  a  few  subscribers  scattered  over  a  comparatively 
15  137 


138  PRINCIPLES  OF  THE  TELEPHONE 

large  area,  as  in  country  districts,  party  lines  are  invariably  used. 
Within  cities,  where  all  lines  run  to  a  central  office,  few  party  lines 
are  used,  and  where  they  are  used  seldom  more  than  four  sub- 
scribers are  connected  to  the  same  line.  Party  lines  can  then  be 
classified  in  accordance  with  the  number  of  subscribers  con- 
nected to  the  same  line,  but  it  is  undoubtedly  preferable  to 
classify  them  in  accordance  with  the  calling  system  used.  We 
thus  have  two  classes  of  party  lines:  namely,  code  ringing  and 
selective  ringing. 

156.  Code  Ringing. — The  most  simple  party  line  system,  and 
the  one  which  was  first  used,  employs  the  code  system.  In  this 
system  all  ringers  are  bridged  or  connected  across  the  line  in 
parallel,  and  all  must  be  of  the  same  resistance  in  order  that  the 
ringing  current  may  be  equally  divided  between  them.  Any 
number  of  telephones,  up  to  about  20,  may  be  connected  to  the 
line,  and  as  all  the  bells  ring  when  ringing  current  is  sent  over 
the  line,  a  code  system  of  ringing  is  used.  A  code  system  with 
which  everyone  is  more  or  less  familiar  consists  of  a  system 
of  short  and  long  rings.  Below  is  a  code  system  for  14  stations, 
with  their  corresponding  numbers. 

Station  No.  1  -  11  -  21  -  31  - 

£       , \.£t       Zi£       '  O^  "  '  " 

3  -  -  13  -  -   23  - 

4  -  -  14  - 
5 

The  central  office  is  always  given  ring  one.  It  will  be  noticed 
that  the  dashes  which  symbolize  long  rings  represent  tens,  and 
that  the  short  dashes  represent  units.  Of  course  in  the  telephone 
directory  only  the  number  is  given.  This  scheme  of  ringing  is 
not  often  used  in  towns  or  cities,  but  is  usually  used  on  country 
party  lines. 

On  local  battery  party  lines  it  frequently  happens  that  sub- 
scribers fail  to  restore  their  receiver  to  the  hooks  or  several 
parties  may  be  listening  at  the  same  time.  In  either  case,  the 
receivers  and  induction  coils  connected  across  the  line  being 
of  low  resistance,  the  ringing  current  passes  through  them  and 
not  through  the  ringer  coils,  thus  preventing  the  central  opera- 
tor's calling  the  desired  party.  To  remedy  this  condition  and 
to  permit  the  receiving  of  a  call  if  receivers  are  left  off  the  hooks, 
a  condenser  is  often  connected  in  the  receiver  circuit,  Fig.  107. 
This  condenser  prevents  the  passage  of  the  low-frequency  ring- 
ing currents  and  causes  them  to  pass  through  the  ringer  coils,  but 


PARTY  LINES 


139 


it  does  not  offer  any  considerable  opposition  to  the  high-fre- 
quency voice  currents.  Hence,  this  condenser  has  the  same  use 
as  in  the  central-battery  system.  It  is  the  practice  on  code  party 
lines  for  one  subscriber  to  call  another  by  giving  the  code  ring 
without  the  call  going  through  the  central  office.  Some  com- 


U 


FIG.  107. 

panies,  however,  desire  to  have  all  calls  originating  on  magneto 
party  lines  come  into  the  central  office  in  order  to  have  a  record 
of  all  calls  made  on  the  line,  and  at  the  same  time  relieve  the 
subscribers  of  the  necessity  of  ringing  parties  by  code.  When 


FIG.  108. 

this  is  the  case,  the  instrument  is  provided  with  a  push  button 
which  may  be  used  to  connect  the  generator  to  ground,  and 
thus  use  only  one  side  of  the  line  for  signalling  purposes.  The 
operation  of  such  a  device  will  be  readily  understood  from  Fig. 
108.  When  the  switch  is  closed  to  ground  the  ringing-current 


140  PRINCIPLES  OF  THE  TELEPHONE 

circuit  is  through  the  sleeve  side  of  the  line  to  the  drop  at  central, 
then  to  ground  and  back  to  ringer.  The  other  bells  on  the  line 
are  not  affected. 

Another  plan  is  to  have  a  direct-  or  pulsating-current  generator 
in  each  subscriber's  instrument.  This  current  has  no  effect  on 
the  ringers  of  the  instruments,  but  operates  the  signals  at  the 
central  office.  The  pulsating-current  generator  is  merely  the 
ordinary  magneto  generator  equipped  with  a  commutator  and  a 
push-button  switch  for  making  connection  with  the  commutator. 
When  the  line  is  connected  to  the  commutator  and  the  generator 
is  turned,  the  current  in  the  line  flows  continuously  in  one 
direction.  It  fluctuates  in  value,  as  shown  in  Fig.  45,  but  does 
not  reverse  in  direction.  Such  a  current  will  operate  the  drop  at 
central,  but  will  not  operate  the  ringers  of  the  other  subscribers. 


FIG.  109. 

157.  Selective  Ringing. — In  selective  ringing  theNringers  are  so 
arranged  that  only  the  bell  of  the  person  wanted  at  the  telephone 
is  rung.  Selective  ringing  is  accomplished  by  two  principal 
methods.  One  is  by  the  use  of  a  biased  bell  with  pulsating 
ringing  currents;  and  the  other  is  by  making  use  of  bells  which 
will  respond  only  to  a  given  frequency  of  an  alternating 
current,  this  latter  method  being  known  as  harmonic  ringing. 

A  common  method  of  selective  ringing,  for  use  where  only  two 
parties  are  connected  to  a  single  line,  is  to  connect  one  sub- 
scriber's ringer  between  one  side  of  the  line  and  ground,  and  the 
other  subscriber's  ringer  between  the  opposite  side  of  the  line 
and  ground,  Fig.  109.  In  order  to  ring  either  party,  then,  it  is 
only  necessary  for  the  central  operator  to  send  ringing  current 


PARTY  LINES 


141 


over  that  side  of  the  line  to  which  the  desired  subscriber  is  con- 
nected, which  will  ring  his  bell  but  will  not  call  the  other  sub- 
scriber. The  talking  circuit  is  connected  across  the  two  sides  of 
the  line,  as  usual.  In  the  four-party  selective  system,  biased 
bells  are  used. 

A  biased  bell  is  a  polarized  ringer  designed  to  operate  with 
pulsating  current ;  that  is,  current  which  flows  in  one  direction  but 
is  interrupted  from  time  to  time.  The  biased  bell  shown  in  Fig. 
110  is  essentially  a  polarized  ringer  with  a  spring  attached  to 
the  armature  in  such  a  manner  as  to  hold  the  clapper  in  the  ex- 


FIG.  110. 

treme  left  or  right  position  when  no  current  is  flowing.  When 
the  clapper  is  in  the  extreme  left  position,  the  right  end  of  the 
armature  is  near  the  right  core  of  the  magnet.  It  is  evident  when 
the  armature  is  in  this  position  that  it  can  be  affected  only  by 
current  flowing  through  the  coils  in  such  a  direction  as  to  cause 
the  left  core  of  the  magnet  to  become  a  S.  pole,  when  it  will 
attract  the  left  end  of  the  armature  and  overcome  the  strength  of 
the  spring,  causing  the  clapper  to  move  to  the  right  and  strike 
the  right  gong.  As  soon  as  the  current  ceases  to  flow,  the  arma- 
ture will  be  returned  to  its  original  position  by  the  spring,  and 
the  clapper  caused  to  strike  the  left  gong.  The  rapidity  with 
which  this  operation  is  repeated  will  depend  upon  the  frequency  of 


142 


PRINCIPLES  OF  THE  TELEPHONE 


the  pulsations,  or,  in  other  words,  the  number  of  times  the  current 
is  interrupted  per  second.  In  order  to  ring  properly,  the  bell 
must  be  in  selective  adjustment;  that  is,  the  spring  must  be 
strong  enough  to  pull  the  armature  back  to  its  original  position 
during  the  time  that  no  current  is  flowing;  yet  the  spring  must  not 
be  so  strong  that  the  force  of  the  magnet  can  not  overcome  it. 
Ringers  not  in  selective  adjustment  can  be  operated  only  by 
alternating  currents.  In  ringing  biased  bells,  selection  between 
four  stations  on  a  party  line  may  be  had  by  connecting  two  biased 
bells,  one  of  each  polarity,  between  each  wire  and  the  ground, 
four  bells  in  all,  as  shown  in  Fig.  111.  When  the  pulsating 


TIP     SIDE. 


FIG.  111. 

generator  is  connected  so  that  current  flows  out  along  the  tip 
side  of  the  line,  the  ringer  at  A  is  operated.  When  the  con- 
nections are  reversed  so  that  the  current  flows  out  through 
ground,  it  will  operate  the  ringer  at  station  B.  In  the  same  way 
the  ringers  at  stations  C  and  D  may  be  operated  by  connecting 
alternately  the  positive  or  negative  terminal  of  the  generator  to 
the  line  and  the  other  terminal  to  ground. 

158.  Harmonic  Ringing. — In  a  harmonic  system  alternating 
current  of  four  different  frequencies  is  used  for  ringing  purposes, 
the  bells  being  so  arranged  that  each  one  will  ring  only  when 
supplied  with  current  at  one  of  the  four  frequencies.  In  order 
that  a  bell  may  ring  for  a  given  frequency  of  current,  its  clapper 
must  swing  from  one  extreme  position  to  the  other  during  the 
period  that  the  current  reverses.  Bells  used  in  harmonic  ringing 


PARTY  LINES 


143 


have  a  spring  which  holds  the  clapper  in  its  middle  position  when 
no  current  is  flowing.  In  order  that  the  ringers  may  operate  at 
different  frequencies,  the  strength  of  these  springs  and  the  weights 


FIG.  112. 


of  the  clappers  are  different.  If  the  ringer  is  properly  adjusted 
for  the  given  frequency,  a  small  ringing  current  will  cause  the 
clapper  to  vibrate  violently  enough  to  strike  the  gongs,  in  the 


FIG.  113. 


same  manner  that  a  very  small  force  at  the  right  time  causes  the 
pendulum  of  a  clock  to  swing.  Just  as  a  considerable  force  is 
required  to  cause  the  pendulum  of  a  clock  to  swing  at  any  but 


144 


PRINCIPLES  OF  THE  TELEPHONE 


its  natural  period,  so  it  is  necessary  that  a  heavy  ringing  current 
be  required  to  cause  the  tuned  ringer  to  ring  at  any  other  than  its 
natural  frequency. 

The  frequencies  usually  used  for  harmonic  ringing  are  16%, 
33%,  50,  and  66%  cycles  per  second.  Since  two  alternations 
are  required  to  complete  one  cycle,  the  number  of  alternations 
per  minute,  corresponding  to  the  above,  are  2,000,  4,000,  6,000, 
and  8,000.  (For  example,  16%  X  2  =  33%  alternations  per 
second;  and  33%  X  60  =  2,000  alternations  per  minute.)  In 
Fig.  112  is  shown  a  Western  Electric  ringer  for  harmonic  party- 
line  service;  and  the  clapper  rods  for  ringers  operating  at  four 
different  frequencies  mentioned  above  are  shown  in  Fig.  113. 


FIG.  114. 

Eight-party  service  may  be  given  by  connecting  four  harmonic 
ringers  of  different  frequencies  between  each  side  of  the  line  and 
ground,  if  such  service  be  desired.  The  ordinary  method, 
however,  is  to  bridge  the  ringers  directly  across  the  line,  as  shown 
in  Fig.  114,  making  only  four  stations  on  the  line. 

159.  Extension  Bells. — Many  times  a  telephone  ringer  can  not 
be  heard  as  far  from  the  instrument  as  the  subscriber  desires, 
in  which  case  an  extension  bell  can  be  used.  As  the  extension 
bell  is  always  connected  to  the  same  line  as  the  ringer  of  the 
telephone,  the  extension  bell  must  be  of  the  same  resistance  and 
have  the  same  adjustment  as  the  other  ringers  of  the  line. 

QUESTIONS 

1.  Into  what  classes  are  party  lines  divided? 

2.  What  is  meant  by  code  ringing? 


PARTY  LINES  145 

3.  What  are  the  advantages  and  disadvantages  of  code  ringing? 

4.  Of  what  use  is  a  condenser  in  a  receiver  circuit,  in  party-line  service? 

5.  What  is  meant  by  selective  ringing? 

6.  In  what  way  is  a  biased  bell  different  from  an  ordinary  polarized  ringer? 
Explain  its  operation. 

7.  What  kind  of  ringing  current  is  used  with  biased  bells? 

8.  Explain  the  connections  and  operation  of  a  four-party  line  using  biased 
bells.     Show  how  each  bell  can  be  rung  without  ringing  the  others.     Show 
connections  by  diagram. 

9.  What  is  meant  by  harmonic  ringing?     What  kind  of  current  is  used  for 
harmonic  ringing? 

10.  How  are  harmonic  ringers  different  from  other  ringers  which  have 
been   discussed?     What   is   the   difference   between   ringers   designed   for 
different  frequencies  of  ringing  current? 

11.  Explain  the  operation  and  connections  of  four-  and  eight-party  har- 
monic lines. 


CHAPTER  XV 
INTERCOMMUNICATING  TELEPHONE  SYSTEMS 

160.  Definition. — An  intercommunicating  telephone  system 
is  the  arrangement  of  several  sets  of  telephones  such  that  any 
station  can  call  any  other  station  without  the  assistance  of  a 
central  operator.  Such  systems  are  extensively  used  in  factories, 
offices,  apartment  buildings,  stores,  and  large  private  dwellings 
as  they  afford  a  ready  means  of  communication  between  different 
departments. 

Telephone  systems  for  intercommunication  may  be  operated 
either  by  a  local  battery  for  the  talking  circuit  and  a  magneto 
for  signalling,  or  they  may  be  operated  entirely  from  a  common 
battery.  When  the  common  battery  type  is  used,  two  sets  of 
batteries  are  invariably  employed. 

The  most  simple  system  of  the  local  battery  type  is  one  in 
which  two  telephone  sets  are  connected  by  a  single  line.  Such  a 
system  needs  no  further  discussion.  However,  when  more  than 
two  stations  make  up  the  system,  the  arrangement  is  more  com- 
plex. Of  course,  all  the  instruments  could  be  connected  to  a 
single  party  line,  but  this  would  necessitate  code  ringing.  The 
usual  arrangement  of  intercommunicating  systems  is  to  have  a 
separate  line  run  from  each  instrument  to  every  other  one  of  the 
system.  For  magneto  ringing  the  circuits  are  quite  simple  and 
easily  designed.  Each  station  is  provided  with  a  panel  upon 
which  are  mounted  as  many  jacks  as  there  are  stations,  and 
lines  running  from  any  one  station  connect  the  jacks  into  as 
many  parallel  groups  as  there  are  stations.  At  each  station  the 
ringer  is  bridged  across  one  line.  This  line  is  designated  at  all 
other  stations  as  belonging  to  the  station  at  which  the  bells  are 
bridged.  The  talking  and  ringing  circuit  at  each  station  is  pro- 
vided with  a  terminal  plug  which  is  used  to  make  connection 
with  the  jack  of  any  other  station.  Fig.  115  is  a  simplified  dia- 
gram of  such  a  system.  When  a  person  at  station  A  wishes  to 
call  some  one  at  station  D,  he  inserts  the  plug  into  the  jack  con- 
nected to  the  J)  line  and  turns  the  magneto.  As  the  only  ringer 

146 


INTERCOMMUNICATING  TELEPHONE  SYSTEMS    147 


that  is  bridged  across  this  line  is  at  station  D  it  is  the  only  station 
that  will  hear  the  call.  As  soon  as  the  person  at  station  D  inserts 
his  plug  in  jack  D,  the  talking  circuit  with  station  A  is  complete. 

Although  such  a  system  is  extremely  simple,  owing  to  the  con- 
venience of  automatic  signalling  provided  by  the  common  battery 
system,  the  latter  is  displacing  it. 

161.  Common  Battery  Interphone  Systems. — Most  of  the 
manufacturers  of  standard  telephone  apparatus  also  manufacture 


STATION  A         I      STATION  &  STATION   C 


STA  TIO'N  D 


iflnf 


^ 


FIG.  115. 

intercommunicating  telephone  apparatus.  In  general  the  prin- 
ciples of  operation  of  the  different  makes  are  the  same,  but  each 
has  some  distinctive  method  of  connection  for  ringing. 

At  each  station  is  a  telephone  set,  either  a  wall  set  containing 
the  keys  and  talking  set,  or  a  desk  stand  with  a  separate  key  box. 
Each  wall  set,  or  desk  set  key  box  has  a  series  of  buttons,  each 
one  numbered  or  lettered  to  indicate  the  line  it  controls.  Typical 
C.B.  intercommunicating  sets  are  shown  in  Figs.  116,  117,  and 
118.  A  person  at  one  station  wishing  to  talk  to  one  of  the  other 
stations  presses  the  corresponding  button  down  to  the  ringing 
position,  and  the  desired  station  is  signalled.  When  this  button 
is  pushed  down,  any  other  button  in  the  set  which  might  happen 


148 


PRINCIPLES  OF  THE  TELEPHONE 


to  be  depressed  is  automatically  restored,  thus  clearing  the 
station  of  any  previous  connection.  When  the  pressure  is  re- 
moved, the  button  comes  back  to  a  halfway  or  talking  position, 
so  that  as  soon  as  the  called  station  receiver  is  removed  the 
talking  connections  are  complete. 

The  wiring  of  an  intercommunicating  system  appears  com- 
plicated, but  this  is  due  to  the  multiplicity  of  wires  at  each 
telephone.  As  a  matter  of  fact  the  circuits  are  quite  simple. 


FIG.  116. 


Diagrams  of  the  circuits  of  two  stations  involved  when  one  calls 
the  other  of  a  Western  Electric  interphone  system  is  shown  in 
Fig.  119.  The  diagram  shows  that  two  sets  of  batteries  are  used, 
one  for  ringing  and  one  for  talking. 

162.  Western  Electric  Intercommunicating  System. — In  the 
diagram  shown  the  station  at  the  left  is  supposed  to  be  ringing 
the  station  at  the  right.  In  doing  this  the  push  button  d  is 
depressed  as  far  as  it  will  go.  This  closes  both  the  ringing 


INTERCOMMUNICATING  TELEPHONE  SYSTEMS    149 


FIG.  117. 


FIG.  118a. 


FIG.  1185. 


150 


PRINCIPLES  OF  THE  TELEPHONE 


circuit  at  d,  and  the  talking  circuit  at  the  lower  contact.  The 
ringing  current  then  passes  from  the  ringing  battery  to  the  bell 
c,  which  it  rings,  at  the  station  called,  through  the  back  contacts 


STATION  NO.  G    LlA/£S' 


CALLINQ    STATION  N&4 


FIG.  119. 


n  of  the  switch  hook  at  that  station,  over  the  wire  s  of  the  line 
and  through  the  lower  contact  of  the  button  d  at  the  calling 
station,  whence  over  the  other  wire  t  back  to  the  ringing  battery. 


TALKING    STATIOH  N0.4 


AN3IV£/r//Hr  ST/lT/0/f  #0.  6. 


FIG.  120. 


When  button  d  is  released,  it  springs  part  way  back  opening  the 
circuit  at  1  but  leaving  it  closed  at  2  and  at  the  lower  contact. 
This  condition  is  shown  in  Fig.  120.  As  soon  as  the  subscriber 


INTERCOMMUNICATING  TELEPHONE  SYSTEMS    151 


152 


PRINCIPLES  OF  THE  TELEPHONE 


CIRCUIT   DIAGRAM    FORA  FULL  METALLIC   SYSTEM 


RINGING  BAT. 


STATION  ; 


TALKING    BATTERY 


RINGING  BATTERY 


OUR  SETS  ARE  WIRED  FOR  FULL  METALLIC 
SYSTEMS. 

TO  ADAPT THtM  FOR  COMMOH  RETURN 
SYSTEMS-MAKE  THE  rctlCWlH*  CMAHit* 
(0  STRAP  TOGETHER   THE  TWO  TALKING 

BATTERY  TERMINALS  (DO  NOT  counter 

THE  TALKING    BATTERY  WIRES  IN  THE 
CABLE  TO  THESE    TERMimAL*) 

(Z)  STRAP  TOGETHER    THE     LOWER 

TERMINALS  OF  ALL  LINES. 
(3)  CONNECT  THE  BLACK  TKAHSPO&IYICM 

WIRE  TO  POSITIVE    (*•)    RlN«lf4CBATTUl 

TERMINAL*.. 

<4) CONNECT  THE  RED  TRANSPOSITION 
WIRE  TO  THE  UPPER  TCRMINAUOF 
THE  HOME  STATION  LINE. 

te)COMNECT  THE  NEGATWE(-)  TALK  I  MS, 
BATTERY  CABLE  WIRE  TO  LOWL* 
LINE  NO.I 


RINGING  BAT. 
TALKING  BAT. 


CIRCUIT   DIAGRAM   FOR  A  COMMON    RETURN    SYSTEM 


REOTKAN*)    i^g         ?»^  FEPTRAMt-4    Vl__ 

,mOHl*4^_  J  POSOWHLM^  ^J 

STATION^  STATION*3 


&%X~tr^C 


TALKING  BATTERY  RINGING    BATTERY 

FIG.  1216. 


INTERCOMMUNICATING  TELEPHONE  SYSTEMS     153 

at  the  station  called  takes  the  receiver  off  the  hook  he  depresses 
the  answering  button  K  which  operation  connects  the  two 
transmitters  TI  and  T2  directly  across  the  line  which  is  composed 
of  the  two  conductors  s  and  r.  The  talking  battery  is  also 
bridged  across  the  line  through  the  two  windings  x  and  y  of  a 
retardation  coil.  The  function  of  this  coil  is  to  prevent  inter- 
ference or  cross-talk  from  other  stations  which  might  be  con- 
nected together  for  conversation  at  the  same  time,  as  the  same 
talking  battery  is  used  for  all  the  telephones  in  the  system. 
The  receivers  R\  and  R%  are  each  connected  in  a  local  circuit 
which  includes  the  secondary  of  an  induction  coil  at  each  station. 
The  connection  between  the  talking  battery  and  the  ringing 
battery  is  necessary  to  prevent  cross-ringing,  that  is,  the  ringing 


FIG.  122. 


of  a  bell  at  a  station  other  than  the  one  called.     Figs.  121a  and 
1216  show  the  arrangement  of  a  typical  Western  Electric  system. 

163.  The  Kellogg  Intercommunicating  System. — The  Kellogg 
intercommunicating  telephone  which  is  shown  in  Fig.  118 
operates  on  the  same  principle,  but  the  connections  differ  some- 
what. Instead  of  employing  the  same  button  for  connecting 
the  circuit  and  ringing,  separate  buttons  are  provided.  Thus 
to  call  a  station  the  button  corresponding  to  the  station  desired 
is  depressed.  This  closes  the  ringing  circuit  but  not  the  talking 
circuit  for  the  closing  of  which  a  separate  green  button  is  pro- 
vided. In  answering  a  call  at  any  station  all  that  is  necessary  is 
to  press  the  red  or  home  button  and  remove  the  receiver  from  the 
hook  in  the  regular  manner. 

16 


154 


PRINCIPLES  OF  THE  TELEPHONE 


164.  The  Monarch  Intercommunicating  System. — The  Mon- 
arch intercommunicating  system  is  also  of  the  push  button 
type,  but  a  modification  is  made  in  the  manner  of  connecting 
the  battery  to  the  talking  and  ringing  circuits.  The  operation 


u  u 

NORMAL-  POSITION 


O  O  O  O  O 
TAUK1NG  POSITIOH 

FIG.  123. 


O  O  O  OO 

RINGING  POSITION 


of  the  system  will  be  readily  understood  from  an  examination 
of  Figs.  122,  123,  124,  and  125.  Fig.  122  shows  the  method  of 
wiring  for  two  stations.  Two  batteries  are  employed,  one  for 
ringing  and  one  for  talking,  as  in  the  Western  Electric  system. 
The  ringing  circuit  is  permanently  connected  to  the  talking 
circuit  at  the  sleeve  side  of /the  line  at 
S;  from  there  it  leads  through  the  buz- 
zer or  bell,  the  lower  contacts  of  the  hook 
switch  to  the  ringing  battery,  and  to 
the  calling  key,  a,  for  station  2  at  station 
1.  When  this  key  is  depressed  the  cir- 
cuit is  closed  through  conductor  Ri  to  S. 
To  call  station  2,  the  person  calling  de- 
presses the  calling,  key  2  which  closes 
the  calling  circuit  as  shown  at  a,  Fig. 
123,  completing  the  circuit  and  ringing 
the  bell  at  station  No.  2.  When  the 
calling  key  is  released  it  springs  back 
part  way  opening  the  ringing  circuit  at 

a,  and  when  the  person  called  at  station  No.  2  takes  his  receiver 
off  the  hook  the  ringing  circuit  is  also  opened  at  the  hook  switch. 
The  talking  circuit  is  controlled  partly  by  the  calling  key  at  sta- 
tion No.  1  and  also  by  the  home  key  at  station  No.  2.  The  switch 
controlled  by  the  home  button  is  shown  in  Fig.  124.  In  the 


FIG.  124. 


INTERCOMMUNICATING  TELEPHONE  SYSTEMS    155 


156  PRINCIPLES  OF  THE  TELEPHONE 

normal  position  of  the  home  button,  the  switch  points  at  d  are 
closed.  This  corresponds  to  the  lower  contact  at  d,  Fig.  122. 
When  the  home  button  is  depressed  at  the  station  called,  the 
switch  points  at  d}  Fig.  124,  are  opened  and  those  at  g  are  closed. 
This  corresponds  to  the  point  g  at  station  2,  Fig.  122.  Normally 
only  one  side  of  the  battery  is  connected  to  the  talking  circuits. 
When  one  station  wishes  to  communicate  with  another  station, 
the  station  calling  leaves  his  home  button  in  the  normal  position, 
but  the  station  called  depresses  his  button,  thus  transferring 
the  battery  connection  at  his  station  to  the  other  side,  bridging 
the  battery  across  the  talking  circuit  through  two  retardation 
coils.  The  circuit  is  not  complete,  however,  until  the  receiver 
is  removed  from  the  hook.  A  complete  diagram  of  connections 
for  ringing  and  talking  between  two  stations  is  shown  in  Fig. 
125.  An  examination  of  this  diagram  will  make  clear  the 
operation  of  the  system. 

QUESTIONS 

1.  (a)  What  is  meant  by  an  intercommunicating  telephone  system? 

(6)  What  is  the  difference  between  an  intercommunicating  system  and  a 
party  line? 

2.  Show  by  diagram  the  connections  between  two  stations  for  the  Western 
Electric  intercommunicating  telephone  system. 

3.  Explain  the  operation  of  the   Western  Electric  system  from  your 
diagram. 

4.  Diagram   the   connections  between   two   stations   for   the    Monarch 
system. 

6.  Explain  the  operation  of  the  Monarch  system  from  your  diagram. 


INDEX 


Action  of  a  condenser,  93 

Alternating  currents,  49 

American  wire  gage,  11 

Ammeter,  15 

Ampere,  15 

Annealed  copper  wire,  table  of,  14 

Arresters,  carbon  block,  122 

lightning,  122 

self-cleaning,  124 
Artificial  magnets,  24 

horseshoe,  25 
Automatic  switch,  68 

B 

Bar  electromagnet,  31 
Batteries,  electric,  6 

primary,  6 

storage,  7 
Battery,  4 

resistance  for  parallel  connec- 
tions, 21 
Bell  or  ringer,  69 

extension,  144 
Bipolar  receiver,  50 
Bridging  telephone,  78 

connections  of,  79 
Brown  and  Sharpe  gage,  11 


Capacity  of  a  condenser,  89 

unit  of,  89 
Carbon  block  arresters,  122 

electrodes,  45 

transmitter,  40 
Cells,  dry,  8 

in  parallel,  20 

in  series,  20 
Circuits,  closed,  16 

electric,  15 

grounded,  17 


Circuits,  local  battery,  78 

magnetic,  28 

of  C.  B.  subscribers7  telephones, 
112 

open,  16 

series  and  parallel,  16 

short,  16 

signalling,  63 
Circular  mils,  11 
Closed  circuit,  16 
Code  ringing,  138 
Coil,  heat,  129 

induction,  4,  59 

retardation,  98 

Common    battery    interphone    sys- 
tem, 147 

telephone,  87,  113 
C.  B.  wall  set,  100 
desk  set,  101 
hotel  set,  100 
Complete  telephone,  72 
Condenser,  88 

action  of,  93 

analogy  for,  92 

and  ringer  in  series,  95 

capacity  of,  89 

manufacture  of,  90 
Conductors,  lightning,  120 

and  insulators,  8 
Connections  of  bridging  telephone, 

79 

Construction  of  electromagnets,  32 
Current,  alternating,  49 

direct,  49 

electric.  14 

sneak,  128 

D 

Direct  current,  49 

receiver,  57 
Dry  cells,  8 
Dynamo,  Faraday's,  63 


157 


158 


INDEX 


E 


Electrical  pressure,  7,  15,  48 
unit  of,  13 

resistance,  9 
unit  of,  13 
Electric  batteries,  6 

circuits,  15 

current,  14 
Electrodes,  6 

carbon,  45 
Electrolyte,  6 
Electromagnet,  30 

bar,  31 

construction  of,  32 

horseshoe,  32 

ironclad.  32 

tubular,  32 
Electromagnetism,  29 
Entrance  holes,  132 
Excessive  voltage,  117 
Extension  bells,  144 


Farad,  89 

Faraday's  dynamo,  63 

Faults,  localizing,  110 

in  C.  B.  telephones,  113 
in  L.  B.  telephones,  110 
on  telephone  apparatus,  107 

Function  of  condenser  in  telephone 
circuit,  95 

Fuses^  126 

enclosed,  126 


G 


Gage,  American  wire,  11 

Birmingham,  13 

Brown  and  Sharpe,  11 

New  British  Standard,  13 

numbers,  11 

Standard  wire,  13 

steel  wire,  13 
Generator,  4,  63 

telephone,  66 
Grounded  circuit,  17 
Ground  wiring,  134 


II 


Harmonic  ringing,  142 
Heat  coil,  129 

Heating  effect  of  current,  117 
Hook  switch,  4,  72 

Kellogg,  73 

Stromberg-Carlson,  73 

Western  Electric,  72,  74 
Horseshoe  electromagnet,  32 

magnet,  25 


Impedance,  59 

Induced  electric  pressure,  48 

Induction  coil,  4,  59,  96 

electromagnetic,  58 

magnetic,  24 

mutual,  58 

self-,  57 

Inside  wiring,  133 
Installation,  132 
Instruments,  telephone,  3,  80 
Insulators  and  conductors,  8 
Intercommunicating  telephone  sys- 
tem, 146 

definition  of,  146 

Kellogg,  153 

Monarch,  154 

Western  Electric,  148 
Interphone  system,  147 

Western  electric,  148 
Ironclad  electromagnetic,  32 


K 


Kellogg    intercommunicating    tele- 
phone, 153 
receiver,  53 


Laws    of   magnetic    attraction    and 

repulsion,  25 
Leading-in  wires,  132 
Le  Clanche  cell,  7 
Lightning  arresters,  121 

location  of,  125 
conductors,  120 


INDEX 


159 


Lightning  phenomena,  118 
Lines,  magnetic,  27 

party,  137 

telephone,  137 
Local  battery  circuit,  78 

systems,  definitions  of,  76 
Localizing  faults,  110 
Locating  faults  in  C.  B.  telephones, 

113 
Location,  of  lightning  arresters,  125 

of  protector,  133 

of  telephone  set,  135 

M 

Magnetic  action,  25 
of  receiver,  33 

attraction   and   repulsion,    law 
of,  25 

circuit,  28 

field,  27 

induction,  24 

lines,  27 

substances,  24 
Magnetism,  23 
Magnetite,  23 
Magnets,  artificial,  24 

horseshoe,  25 

natural,  24 

permanent,  26 

temporary,  26 
Magnet  wire,  33 

Manufacture     of      telephone     con- 
denser, 90 

Measurement  of  wire,  10 
Microfarad,  89 
Mil,  circular,  11 

Monarch    intercommunicating    sys- 
tem, 154 
Mutual  induction,  58 

N 

Natural  magnets,  24 
Nonconductors,  9 
Nonmagnetic  substances,   24 

O 

Ohm,  13 

Open  circuit,  16 

Operator's  receiver,  55 


P 

Parallel  cells,  20 
circuits,  16 

resistance  of,  17 
connections,  battery  resistance 

for,  21 

Party  line,  code  ringing,  138 
harmonic  ringing,  142 
selective  ringing,  140 
lines,  classification  of,  137 
Permanent  magnets,  26 
Power  circuits,   protection  against, 

125 
Pressure  and  resistance  of  electric 

current,  15 
electrical,  7 
Primary  batteries,  6 
Properties  of  sound,  36 
Protection,   against  power  circuits, 

125 

against  weak  currents,  128 
of  telephone  lines  and  appara- 
tus, 117 
Protector,  127 

location  of,  133 
Western  Electric,  127 

R 

Receiver,  4,  50 

action,  23 

and  transmitter  in  series,  95 

bipolar,  50 

direct-current,  57 

early,  48 

Kellogg,  53 

Monarch,  57 

operator's,  55 

sensitiveness  of,  55 

Western  Electric,  51 
Reluctance,  28 
Resistance,  unit  of,  13 

electrical,  9 

of  a  parallel  circuit,  17 

of  a  series  circuit,  17 
Resistivity,  9 
Retardation  coil,  98 
Ringer,  4,  69 
Ringing  code,  138 


160 


INDEX 


S 

Selective  ringing,  140 
Self-cleaning  arresters,  124 
Self-inductance,  58 

induction,  57 

Sensitiveness  of  receivers,  55 
Series,  cells,  20 

circuits,  16 

resistance  of,  17 

telephone,  76 
Short  circuit,  16 
Shunt,  16 
Side  tone,  107 

wiring,  112 
Signalling  circuits,  63 
Sneak  current,  128 
Solenoids,  29 
Sound,  35 

loudness,  36 

pitch,  36 

properties  of,  36 

timbre  or  quality,  36 

velocity  of,  36 

Sources  of  excessive  voltage,  117 
Speech,  transmission  of,  37 
Subscribers'  telephones,   circuits  of 

C.  B.,  112 
Substation,  130 

instruments,  111 

faults  in,  111 
Switch,  automatic,  68 

hook,  72 

T 

Table  of  annealed  copper  wire,  14 

of  resistivities,  9,10 

of  wire  gages,  12 
Telephone  batteries,  7 

bridging,  78 

circuit,    function    of    condenser 
in,  95 

common  battery,  87 

generator,  66 

instruments,  3,  80 

Kellogg       intercommunicating, 
153 

lines,  137 

locating  faults  in  C.  B.,  113 

operation,  1 


Telephone,  protection  of,  117 
receiver,  23,  48 
series,  76 
set,  72 
desk,  83 
hotel,  82 
location  of,  135 
wall,  80 

subscribers',  112 
systems,     intercommunicating, 

146 
troubles,  106 

localizing  of,  106 
Temporary  magnets,  26 
Tests  for  telephone  troubles,  106 
Tone,  side,  107 
Transmission  of  speech,  37 
Transmitter,  3 
carbon,  40 
Kellogg,  44 
Monarch,  45 

new  Western  Electric,  42 
operator's,  45 
White  solid-back,  40 
Tubular  electromagnet,  32 

U 

Unit  of  electrical  pressure,  13 
of  resistance,  13 


Variable  resistance,  38 
Velocity  of  sound,  36 
Volt,  13 
Voltage,  117 
Voltmeter,  13 

W 

Weak  currents,  protection  against, 

128 
Western  Electric  protector,  127 

receiver,  51 

system,  148 

Wheatstone's  bridge  connection,  99 
Wire  gage,  table  of,  12 

magnet,  33 

measurement,  10 
Wires  leading-in,  132 
Wiring,  ground,  134 

inside,  133 


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